What's the 3rd order intercept point?

it is the point at which the fundmental power equal to the IM3 power

Added after 4 minutes:

note this is usually a theoritical point as the output power will compress before reachng this point and hence the IP3 is found by intersecting the extention of the fund. and IM3 components "i.e. u linearize the system around some small signal operating point " ,usually it gives an indication about linearity where usually P1dB=IP3-10

pls write relations for computing IP3 or graphes for it

refer to any RF book, i recommend Razavi's and Thomas Lee's for a start