From a socio-economic perspective, The Gap and Fig Tree Pocket are both very nice areas; Forestdale is in a much more socially disadvantaged part of Brisbane.
Personally, I think Fig Tree Pocket is just divine, and one of the most gorgeous suburbs in Brisbane. It's handy to Indooroopilly Shoppingtown, the Western Freeway (easy commute to city), has frequent buses through most parts (anywhere near Fig Tree Pocket Rd or Jesmond Rd should be fine), is near many good public and private schools, and has many beautiful parks and the river. All in all, it's a gem.
I'm sure there are people who could wax lyrical about The Gap, too, but I'm more of a south-west kind of gal
, so I'll leave that to somebody else. My perception is that road traffic to the city from The Gap is more challenging than from FTP, and there's no big shopping centre nearby (that I can think of). And no river. But it is closer to the bushwalking parks.
Personally, I'd forget Forestdale. The area is very industrialised and increasingly so, I would be surprised if public transport is any good, and it's just not a very beautiful part of Brisbane. Sure, there are beautiful individual properties on acreage with bushland surroundings, but the bigger picture of your neighbourhood isn't as pretty as in the other two areas.