I have just restrung my acoustic but can't seem to tune my third string back to G (standard tuning). The best I can do is to tune upto an F but after that the string just breaks (tried 3 times already). I know I can tune an octave lower but the sound is pretty cr1p. Can anyone suggest what else I can do?

At the saddle or does the eyelet break off?  Check the saddle for a groove or a chip or sharpness at the back of the saddle.
Wierd.  Are you sure you're not trying to tune it an octave too high?
Yea, you have to be careful, the 3rd string at the 4th fret should be the same note as the 2nd string open.
Although I've been playing a long time, I don't have a good sense of relative pitch. When restringing, I like to use a reference pitch and "walk" the string pitch up to it. That is, I start obviously low, then find the fret with that pitch, and tighten the string until the reference pitch matches the open string.