Normal 3 pin plug can be used run 1HP air-con without problem. Higher power need to use 15A socket.

If not mistaken a 20A MCB is used for 2x13A sockets. 1HP aircon is using no more than 1kW. So, each MCB can support 2 x 1HP aircon without problem. If you need to add 1 more 1HP aircon, just make sure the power is taken from another MCB, i.e. using 3-pin socket on separate circuit.

As long as total current usage is less than 63A (power rating for 1 phase), there is no need to use 3-phase supply.