DEO - what abernova said - but taking your question literally: A 3g accelaration would be approximately 30 ms-2, and if this is applied to an object at rest you can easily calculate what speed you'd be at various times going forward, so at t=1s, v=30 ms-1, t=5s 150 ms-1, t=10s 300 ms-1 etc...
Of course with those sorts of accelerations if applied constantly to your superhero and say you are flying through the earth's thick atmosphere, she should at some point (quite soon) in her flight cause so much friction with the air that she'd look like a fireball blazing through the sky
Given your discussion re: her costume getting cut up by glass, then perhaps such a action would completely incinerate her costume off her
