Easiest way to start is use one monster of the same level of the party per player. The Monsters by Level section at the back of the MM will help you find monsters of the right level.
For example: A battle for 3 level 1 players could consist of:
Four minions count as one monster. An Elite monster counts as two normal monsters. And Solo monsters count as 5 monsters.
- 1 Kobold slinger (lvl 1 Artillery)
- 2 kobold skirmishers (lvl 1 skirmishers).
For the above example: You could replace one of the skirmishers with 4 kobold minions (lvl 1 minions), so the encounter would be:
The next level of complexity is to actually use an Experience "Budget." The encounter table on pg 57 of the DMG can be easily extrapolated to 3 players. When you pick the level of encounter you want (equal to level of players if you want a simple challenge, up to a few levels above the level of the players if you want them to sweat a bit), you use the budget value and select monsters who's total experience point rewards equal the budget.
- 4 kobold minions
- 1 kobold skirmisher
- 1 kobold slinger
Looking at the table, extrapolate down, and a lvl 2 encounter for 3 players would be 375 xp. Here's a sample breakdown of Goblins with that budget:
- 1 Goblin Skullcleaver (Lvl 3 brute): 150 xp
- 1 Goblin Sharpshooter (Lvl 2 artillery): 125 xp
- 4 Goblin Cutters (Lvl 1 minions): 4x25xp = 100xp
- Total: 375 xp.
This will get you started... there are plenty other levels of complexity, but we'll leave it there for now