For simplicity’s sake: 
D = dogs, C = cats, R = rabbits
Since the dogs and the cats are the same number, we can say C = D. 
R = 1/4D
2D + 1/4D = 36
Combine like terms and get the same denominator
9/4D = 36
9D = 144
D = 16
Now we know that D is 16, so C must be 16 as well. That leaves us with 32 cages. 
36 - 32 = 4 rabbit cages. To double check this, 1/4 of 16 does equal 4. 
Final answer: 16 D, 16 C, 4 R