I always wondered that too. I think I tried it and it only worked for strictly monsters, traps, and spells. Doesn't work with effect monsters, different leveled monsters, ritual, etc...
From my experience at least..
So wait Joey, Weevil,Rex,Odion, and Tea can't get it?
Spell and Trap are in the same category. So you would get 3 monsters, 1 spell or trap most of the time. But since the monster:spell&trap ratio is not exactly 3:1, it is possible to get 2 monsters, 2 spell/trap.
Sorry but. Can confirm spell and traps are different category. 10 duels with 13 monsters, 5 spells and 2 traps. Starting hand ALWAYS either 3 monster + 1 spell or 2 monster + 1 spell + 1 trap. I never got 3 monster + 1 trap.
If spell and trap in the same category, then 3 monster + trap is possible but, no
Deck 0%Monster(M) 100%Spell/Trap(S/T)>4xS/T
Deck 25%M 75%S/T > 1M 3 S/T
Deck 50%M 50%S/T > 2M 2 S/T
and so on
Deck 80%M 20% S/T > 1M 2S/T 1Random