Having had a lot of experience with young horses, I would be inclined to think at this point in her training it's a not-so-happy coincidence that she happens to want to roll and you're asking her forward. I think it's unlikely that it's a learned avoidance tactic.... yet. I think it sounds like she doesn't really know what forward cues mean. You're right, she will learn to do it as avoidance reasons soon enough if it works for her tho.
You say she's 'sour' to go forward? This usually implies that a horse has had enough of some monotonous, unrewarding behaviour, or is nervous of it - such as leaving the home environment or friends. As a 3yo baby with very little training under her belt, I think it's far more likely that she just doesn't understand what's wanted. She may have been taught to yield well at the breaker's yard, but horses don't generalise well & that small experience in that particular environment doesn't mean she 'knows' the 'commands' in different situations.
It sounds like you first need to do more work with her in teaching & reinforcing yielding(moving from pressure with softness & understanding, not resisting or escaping it), especially to teach her to move forward on cue. I would teach her on the ground first, to yield in all ways - eg. forward, backwards, right left, up, down, and from various different cues & positions - eg. fingers on her side, where your legs will be when riding, rope/reins, rope or stick swung behind her, etc. I would also reinforce these lessons in a variety of different settings - eg. take her for a walk & have a play out on the trail somewhere. Then I would go over all those lessons on her back, starting in an arena or some familiar, low distraction area.
Once she's pretty good at yielding, take her out on the trail & give her your forward cue when she thinks about rolling. If she ignores it & does more than thinking about it, don't be afraid to give her as strong as necessary 'forward cue'! But start softly & escalate the cue gradually, if/as necessary. Keep the pressure up until she stops thinking of rolling, even if the first time or 2 this means you've had to jump off her & she's down with you still smacking her rump or such. Quit hassling her the *instant* she starts to respond, even if the first time or few this is just a hesitation.
If you've first taught her how to respond appropriately, making this easy and pleasant for her, then you make the 'bad' things consistently unpleasant, she'll soon stop doing those things, because they don't 'work' for her.