I used to lift weights every other day. But 2years have passed and I've probably lost most of what I gained in 1year of lifting. If I started back, I would only want to lift on Friday, Saturday, and Sunday. How effective would that be? Should I even bother with it?
Yes. Any exercise is more effective than no exercise.
Depending on how much you used to lift, it will come back quicker than it would in an untrained person. Part of the lifting process is neural adaptation as well as other physiological adaptations that don't really disappear.
It will still take a while to get back to where you were, and you will still feel soreness as you get back into it, but it will take less time than it did initially, and will be better for you than doing nothing.
As JohnP said. Yes! I would like to add if you are stacking days that it may be wise to switch between primary muscle groups on alternating days. Especially when starting back.
It may be tempting to get full body workouts each day since you have only the 3 days, but it could lead to injury.
If it were me. I would alternate the primary muscle groups on day 1 and 2, and have a fun full body workout to finish it up on the last day. I'd even include a little uptempo strength routine on that last day to get that heart going!
Actually, this answer can vary. If you are lifting 3 days in a row and are doing only upper body as you say in your comment and are working on the same muscle groups day after day, then your muscles are probably not getting enough time to rest to grow back stronger in between your workouts. However, if you are doing the bench press on Friday, curls on Saturday, and "nose breakers" on Sunday you are getting enough rest but may not be exercising each individual muscle group enough; not to mention that nose breakers can cause elbow tendonitis. You should be training each muscle group at least 2 times a week. And not to even mention, where are your leg days? Legs and glutes are the strongest muscle groups on the human body, so why would you deny them? Overall, I think you need a real workout plan. Try something like this: Monday: Full body workout(chest, back, legs, biceps, deltoids) Wednesday: Full body workout Friday:Full body workout And take the other days for rest. Don't worry about working abs individually, as they will be worked through squats and deadlifts. This way you are exercising your entire body 3 days a week and giving it time to heal back up.