Comparing her to the Wooly Thoctar, the costs should be fine, IMO. [link] Three-color cards usually get a big boost, and legendary creatures a little extra in compensation for the drawback; flying and trample are also a little redundant, which would lower the cost. Then again, I'm the guy who made her a 3/2 flying, first strike, super-haste for RRW, so I'm biased.
Could probably stand to cost one or two more, but have an alternative way to get her to table faster - say, akin to Qasali Ambusher? [link]
2URG // 3/4 Flying, Haste, Trample When a creature is attacking you or a planeswalker you control, and you control a Mountain, Forest and an Island, you may play Rainbow Dash as though it had flash and without paying its mana cost.
...I just realized that you could play both Runes of the Deus and Clout of the Dominus to her. Can you say "ouch"?
This is undercosted. There are a total of four creatures with p+t of 3 or more, CMC of 3 or less and Haste. One can't block and two are sacrificed after they've attacked. The last is a red/green with Wither. There are more 3/3 CMC3 flyers, but most have drawbacks. The same applies to tramplers. Some, but most have drawbacks.
All three abilities on one 3/3 for 3, when the pony sets have lots of multi-colour love? This is ferociously undercosted. The easiest way to fix it would be to make Dash a flyer and something like Flash. Something that puts Dash into play blocking target creature, maybe, as she's only seen using the Rainboom when she's defending her friends?
I'm no wording or custom ability balance guru, but maybe something like Reverse Ninjutsu: Return a creature to your hand, put Dash into play blocking target creature, dash gets -X/+X (enough to give 0 power but high toughness) until end of turn? There should be a mana cost attached to that as well, possibly some wording allowing that to be done during the declare blockers phase.