You have a pair of 50 amp breakers, for 100 total. It is a two leg system. The right column of breakers is one leg and the left column the other. Because you have three A/C units, two will be on one side and one will be on the other.
Each A/C will draw about 13 AMPs with peaks to about 20 as the compressor cycles. What else is on the leg that has two A/Cs on it?
Can you tell which of the two 50 amp breakers is tripping? Can you monitor loads on your Aladin screen?