I normally use the CR / XP equivalent
So simply summ up all the enemies XP (remember to combine multiple monster of the same type to one "CR Monster") and look which CR is nearest.
So in your case
CR 7 3200 XP
CR 5 1600 XP
4x CR 3 = CR 7 = 3200 XP
Total 8000 XP which is around CR 9-10
For XP I normally simply award the XP for each enemy on it's own, so in your case the group would gain (3200 + 1600 + (4x 800))XP = 8000 XP if they managed this encounter.
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