What a great problem you have. Most people getting into this game don't have a rifle and are trying to choose one. You have three and want to know which one to start with....short answer, work with ALL of them! You have two great platforms from which to start.
1. 308 - awesome long range rifle and it will take 5000+ rounds before the barrel is burned out. Loaded right, (take Michael's advice here) the heavier bullets will carry the energy better to 6-800 yards. Personally, I shoot 175vld's in my 308. It 1000lbs of retained energy to 675 yards.
2. 270wsm - another awesome long range caliber. Loaded with 140grain bullets at 3200fps, this bullet has 1000lbs. of retained energy to 800 yards. The downside of the short-mags is that they are barrel burners.
(as a rule of thumb, I consider a mininum of 1000lbs. of retained energy the benchmark of energy required to cleanly kill deer with a well placed shot)
3. Ruger 7mm mag. That 7 mag is an awesome cartridge, it's just in the wrong rifle. I agree with earlier advice, sell this rifle because of the limited aftermarket parts available. Then use the funds gained from the sale to improve your remaining two rifles.
Have fun.