First of all: don't. You'll hurt your yonma game by playing sanma too much. If you really want to persist (having a realistic shot at getting into the top 100 players sure looks attractive, huh?), then keep reading.
There are four changes that you need to be aware of when playing sanma.
- There is no chii. You cannot chii from people, but pon and kan all work as expected.
- Manzu are gone, except for 1m and 9m.
- Tsumo loss is active: The scoring works as normal, but the fourth player who should pay on a tsumo does not exist; therefore, a tsumo is worth less than ron.
- There is nobody sitting at the North Wind (pei), ever; consequently, there is no pei yakuhai.
Let's look at these changes in detail.
There is no chii. You cannot chii from people, but pon and kan all work as expected.
No chii means that you cannot take advantage of sequences inyour hand quite that easily. Open tanyao becomes substantially more difficult. Your only calls are pon and kan. This usually makes it not worthwhile to call at all unless it's for a yakuhai triplet or you're on path for a honitsu or chinitsu.
Manzu are gone, except for 1m and 9m.
Manzu being gone except for 1m and 9m means that these two tiles are the epitome of worthless. While they can still form a pair and a triplet, they're not honors and thus unusable as the honor parts of a honitsu. Your only way of getting a honitsu of manzu is by having two triplets of 1m and 9m plus two sets and a pair of honors. Similarly, they get in the way of making honitsu or chinitsu of the remaining actual suits. They do, however, make very good trick waits if you can set them up; nobody expects a wait on 1m or 9m. Finally, 1m and 9m still function normally in the context of kokushi. Usually, however, they're just trash tiles in your hand; even if you start with a pair of them, you'll usually get a more useful pair later.
Tsumo loss is active: The scoring works as normal, but the fourth player who should pay on a tsumo does not exist; therefore, a tsumo is worth less than ron.
Because of tsumo loss, ron is always worth more than tsumo. Consider a mangan. In suuma, a mangan is worth 8000 points (dealer 12000 points); on tsumo, this is split into either 2000 points for two people and 4000 points for the dealer (or 4000 all if dealer). In sanma, a mangan ron is worth 8000 points (dealer 12000 points), but a mangan tsumo is only 6000 points (dealer 8000 points) because the split is still made the same, but the third person doesn't exist.
There is nobody sitting at the North Wind (pei), ever; consequently, there is no pei yakuhai.
Pei can still serve three functions: It can form a pair (this pair is always otakaze, so can be used with pinfu), it can form a triplet (but isn't yakuhai) including the ability to be kanned, and you can call pei with them. When you call pei, you set the tile next to your hand and draw a new one from the dead wall. The pei that was set aside counts as kita dora. If the dora indicator is shaa (west wind), the pei tile will also count as normal dora. Calling pei does not count as opening your hand, so you can riichi normally with it. You do not gain any fu from doing this. If you win by drawing the replacement tile from calling pei, you are awarded with the rinshan kaihou yaku. You can also ron people calling pei if you're waiting for a pei, but this does not award chankan. Calling pei also interrupts tenhou, chiihou and double riichi, but does not interrupt ippatsu.
Other than this, sanma is played like suuma. However, these changes make for a massive difference that will lead to first timers usually getting their shit wrecked even if they are good in suuma.
The above changes lead to some changes in the usual strategy for suuma. While most of Daina Chiba's Riichi Book 1 still applies, you should be aware of the following challenges and opportunities.
Hands below mangan are somewhat rare, especially in lower level play. Since every pei awards a kita dora, which means one extra han, you can already wage a guess that an opponent with two pei next to their hand will likely have a mangan (2 fu already guaranteed, plus whatever they have in their hand) already. Do not panic when you get hit by a haneman early on; you can make this back decently enough.
Since there are less tiles, hands tend to be made much more quickly. If you are not in tenpai by turn 6 (i.e. at the end of the last row), strongly consider folding. Someone at the table is usually in tenpai by then, and if it isn't you, then you're likely to be the one dealing in. Similarly, riichi after turn 9 (middle of the second row) is very unlikely to fill; re-evaluate your position every turn and consider folding instead if things start looking dangerous.
This is aggravated by another circumstance: Because of tsumo loss and scoring, riichi is often not very attractive: The difference between a haneman tsumo and a mangan ron as non-dealer is just 1000 points (9000 points haneman tsumo, 8000 points mangan ron)! And within the tiers of haneman, baiman and sanbaiman, sometimes the single han from riichi makes no difference at all. Therefore: Unless your score is below haneman or you have a trick wait, stay dama! The uradora aren't worth the inability to fold.
Despite what I said about folding and pei being a very, very safe tile, it is usually more useful to get a replacement tile by calling pei instead of holding on to it as a safe tile or trying to make a trick wait based around it; usually, the elaborate strategies end up not working out.
And now you know all there is to know about sanma. Go forth and hit that leaderboard!