EA-6B1 - I too, am just beginning to recognize the differences between all the various aircraft, and while obviously you will be able to do it in an instant after gaining more experience, watching videos, being around them, and the like... it's nice to have a couple easy rules to distinguish them quickly.
First of all, in my opinion the F-14 is the easiest of the three to tell apart because while they are all dual engine... the F-14 is the only one with the two engines set several feet apart, just look at the rear of the jet and if there is a gap between the engines, that's the F-14.
Both the F/A-18 and the F-15 have their engines directly next to each other. As JOEAE2005 mentioned, the vertical stabilizers in the rear of the 18 are angled, whereas the f-15's are parallel. If you can't make that out, look at the leading edge of the wing. As jboomer mentioned, the F/A-18 has the extended strakes that lead into the wing, where as the leading edge of the F-15 wing is interrupted by the square jet intakes next to the fuselage.
Here are links to a few pictures that will help you see these differences.
http://militaryphoto.com/llinares/llinaresf14.html
http://militaryphoto.com/llinares/llinaresf15.html
http://militaryphoto.com/llinares/llinaresf18.html
I hope this helps clear up some of the confusion.
~Eric