I am constructing a residential hotel. On first floor toilets are designed near the entrance of the room. Drainage of the toilet has to travel approx 15 feet on floor. Can I use 3inch PVC pipe for the drain in-steed of 4 inch (routinely used) to minimize the hight of floor?

What country?
It's hard to believe that you would be building a hotel in the US without your building inspector signing off on this already.

In the USA, you can install up to 3 toilets/bathroom groups on a 3" building drain.

However, to be absolutely sure this is accepted in you area, you would want to talk to the local plumbing inspector. I would not suggest just going with what I say here online... just imagine if you were told this was no good at a later date!. UGH!

Consult local people, but sounds reasonable from here!

Good luck!

Mark