Check the valve stem. Take the cap off and use either the spray bottle method mentioned in previous posts or you can simply spit in your finger tip and wipe that on the end of the valve stem. If it bubbles, replace the valve - problem solved.
Also, be sure and check the bottom of the valve, (the rubber part that goes into your wheel). Sometimes those leak as well, especially if they are old. (I have a trick to fix those that involves carb cleaner, but I'm afraid to share as it also involves fire). If it's the valve stem leaking, you can try twisting it back and forth and sometimes it will re-seat. Be careful not to twist too much or too hard as they will break and now you're breaking your bead and putting in a new valve stem.
It might also be leaking around the bead. Be sure and check there as well.
Three pounds a week is not a huge leak, (not like a hole that makes your tire go flat in five or ten minutes - that's what most shops deal with and so they're used to looking for a big stream of bubbles when water testing). Give it plenty of air, (not too much, we don't want you to blow up). If it's a 35 psi, put in 45, it will make the leak easier to see when you hit it with the soap mixture. If your wife will let you, put it in the bathtub - that's what I do. My wife loves me.
