can a bad mattress 3ys old cause circulation problems in the legs and feet?

Have no idea. If you can't afford to replace the mattress, try placing something in the bed that will elevate your legs for better circulation. You could invest in a mattress topper for better general comfort. Research first which toppers are recommended by the public.

Unlikely, look for another cause. See your doctor.

How bad is this mattress? Does it bend in weird ways? I doubt it's the mattress but it could be, if your body isn't fully horizontal while you're lying down. That's a problem.