rotten egg gas H2S can be destroyed by reacting with oxygen according to the equation:
2H2S + 3O2 --->2SO2 + 2H2O
how many moles of which reactant will be left over after reacting 3 moles H2S with 4 moles O2

Which reactant is in the majority here? You should be able to work that out by looking at how the moles of each substance are used up in the reaction.

What do u mean which reactant is the majority

Well your H2S reacts in the proportion 2:3 with the O2. So you shoul dbe able to use this ratio to work out which you have more of in respect to this equation.

How do I work that out

Here is your question split up into steps. You should be able to do all of these steps easily from your notes:

How many times could you perform the reaction using 3 moles of H2S?
How many times could you perform the reaction using 4 moles of O2?

Which of these is less and therefore the limiting reagent?
Therefore how much of the non-limiting reagent do you have left if the reaction is performed the amount of times that it can be for the limiting reagent?