Only using them once a week for 3mins isn't going to do anything unfortunately. I was quite pale and had to go on for two weeks, starting at 4mins and working my way up to 8mins, before I saw any difference. You've got to keep going for a while after that too in order to top up and keep your tan.
I went on them almost everyday for 3 months and it's the stupidest thing I've ever done. I started getting all these raised lumps on my stomach and so went to my doctor. I got a free skin consultation under a UV lamp and I couldn't believe the amount of irreversible damage I'd done to myself in such a small amount of time. I've found a brilliant fake tan called St Moriz, which is cheap and absolutely amazing, definitely the best one I've used. Give that a try instead, once you begin tanning on sunbeds it's very, very hard to stop and I wish someone had prevented me from even starting.