HI Amit,
If you worked only for a very small stint in A(1,2,3,4 month).. you can hide above 5 months gap will be questioned, otherwise, I have few questions for you here:
1..What is the harm in sharing the details?
2..How many years you worked in "A"?
3.. Are you prepared with justification of the duration for your "A" company. (1/2/3 years), if you are hiding.?
4..What if "C" ask you to submit all relieving & Exp. letters of pervious employers during on boarding ?
Regards
Piyush
Manager HR-Pune