Hi T.,
First of all, don't despair. Sometimes kids are late bloomers. And sometimes kids will entirely skip the crawling stage and move into walking. In the meantime, there are some things you can do that will help your baby whether she crawls or not.
When she does have those 3-5 minutes of tummy time, engage her in really active play. Talk to her and rattle her favorite toys in front of her. Another thing you can do is to get a medium size beach ball and put your baby tummy down on top of the ball. As you hold her, slowly roll the ball forward until her hands touch the ground, then roll backwards until her feet touch the floor. Do this a few times to get her used to putting weight on her arms. The babies generally like this activity. But when she starts getting fussy, it's time to move onto something else.
As far as the rolling, when she is on her tummy, try positioning her hands and arms under so her head is up. Then take a rattle and shake it above her head so she sees it and move it in the direction you want her to roll. Hopefully, she will continue to want to see the rattle and her head and body will turn. You can provide her with help by moving her hips in the direction of the roll.
Good luck. I'm sure your baby will get it. When she is walking around a year from now, this will be a story you can file away to tell her about when she is older. :-)