If 3 different planes ask for clearance to land at the same time and are doing different(sic) how do you decide who lands first?
Well, as a radar man, I'd say to number one "XXX123, keep the speed up, route direct the centrefix r/w 27 number one." To the second, I'd slow him down a little and the farthest away would be popped in at the end.
As a tower man, I'd clear the one in front to land, then the second (once no1 was clear of the drag), then the third (ditto), since radar will have kindly sorted your landing order for you.
Good luck with it.