Answer:
we 1st can get the weight of rat by 
1 rat and 1 cat + 1 dog and rat = 30
 2 rat + 1 cat + 1 dog = 30 
Then 1 rat and cat measure 24 so 
 2 rat + 24 =30
2 rat + 24 =30 1 rat = 3 kg 
2 rat + 24 =30 1 rat = 3 kg 1 cat + 1 rat = 10
2 rat + 24 =30 1 rat = 3 kg 1 cat + 1 rat = 101 cat + 3kg = 10
2 rat + 24 =30 1 rat = 3 kg 1 cat + 1 rat = 101 cat + 3kg = 101 cat = 7kg and
2 rat + 24 =30 1 rat = 3 kg 1 cat + 1 rat = 101 cat + 3kg = 101 cat = 7kg and 1 dog + 1 rat = 20 kg 
2 rat + 24 =30 1 rat = 3 kg 1 cat + 1 rat = 101 cat + 3kg = 101 cat = 7kg and 1 dog + 1 rat = 20 kg 1 dog + 3kg = 20 kg
2 rat + 24 =30 1 rat = 3 kg 1 cat + 1 rat = 101 cat + 3kg = 101 cat = 7kg and 1 dog + 1 rat = 20 kg 1 dog + 3kg = 20 kg 1 dog = 17kg 
so we get the weight of each now we r going to sum them 1 rat + 1 cat + 1 dog = x
1 rat + 1 cat + 1 dog = x 3 kg + 7 kg + 17 kg = x 
1 rat + 1 cat + 1 dog = x 3 kg + 7 kg + 17 kg = x 27 kg = x ..... is the mass of 3 of them