How to compute total interference power at receiver antenna
The proper way to sum the interference is to sum them in power, unless they are coherent, in which case they would sum in magnitude. Coherent would mean the interference are all at the same frequency and locked in phase. This is unlikely so if not state explicitly you would assume the are non-coherent and a power sum should be used.
To sum them in power you would simply add the powers and then convert the sum to dBm. If starting from dBm, you would convert them to power quantity first, add and then convert back to dBm as demonstrated below using the OP's quantities:
0.4 mW is -3.98 dBm
0.035 mW is -14.56 dBm
0.2 mW is -6.99 dBm
The total power would be 0.4 + 0.035 + 0.2 mW = 0.635 mW which is $10\log_{10}(0.635)= -1.97$ dBm.
If we started with the dBm quantities we could compute this same result as follows:
$$10\log(10^{-3.98/10}+10^{-14.56/1}+10^{-6.99/10})$$