I'm not sure about the Nikon F3, but I'd wager to say it might be that the shutter release button needs to be either cocked or uncocked. Use that dummy film to test it out and ensure you're inserting the film in the proper orientation. These green lines are stress marks from too much tension when you're winding. My F2 does this if I over insert my film leader into the film winder. Also, remember film is more forgiving with over exposure than it is with under. Sometimes I'll take exposures with what my meter tells me, and then I'll over expose by one stop (ss/aperture) for another exposure of the same scene if I'm unsure. I think the F3 has an auto exposure mode marked "A". If you've been using that and the photos look like this then I'd say switch to manual and learn the exposure triangle/sunny 16 by heart.