How to I convert 1phase load into 3phase load while deciding the sizing(rating)of distribution transformer

A basic formula is P=IE*1.73.

P= Power
I= Amperage
E= Voltage
1.73= 3 phase constant

Once you arrive at the actual total 3 phase power, increase by 25 % to size the transformer.

What exactly are you trying to do?

Distribution transformer??
How are you going to convert single phase into 3-phase using a transformer?

Besides, this is FAR beyond the scope of DIY work. If you have to ask questions like this you should NOT be working on such things.