So I'm going to drop some general observations here and assume that they apply to you. It should go without saying that I don't know anything about you other than what you've said above, so I could be way off.

Key assumptions:

1. Your form is off.
2. You have weak hip and core musculature.
3. Your posture sucks.
   

Key heresy:

1. Modify Starting Strength

So you're new to lifting. Great! Welcome aboard. You are now no doubt identifying several aspects of your body that are weaker than others. This is good - it sucks - but learning this this good.

If your lower back hurts during deadlifts, you're not doing them right. Certainly your lower back is involved in the lift, but it's most likely picking up the slack for weak hamstrings and glutes.

Couple that with heavy squats and OHP and your lower back is going to take a beating. If you're falling too far forward on your squats your lower back is going to fight for better positioning. If you're going into extreme anterior pelvic tilt (APT) on the OHP, your lower back is going to get fried.

Now add to this a heavily sedentary lifestyle outside the gym and you're just fucked all around.

Luckily, the fix to all those assumptions above is the same, because they are all aspects of the same problem.

Keep working on your form. Practice, practice, practice. But remember to not become a form nazi. Form is important, but it's not the most important thing, ever, zomg don't go heavier if your form is not perfect rabble rabble rabble.

Strengthen your hips and core. I suggest you add barbell hip thrusts and stiff leg good mornings to your routine. There are other good ones - kettlebell swings (a dumbbell will work), Romanian deadlifts, glute ham raises, Russian leg curls, etc - but you don't need to do them all. 3x10 is plenty of work, you don't need to over do it. If you want, cycle through them occasionally if only to see which ones you like. A word of caution on the good mornings - go light. Again, you should not feel these in your back, but in your hamstrings. Also train your core, but for the love of god be smart about it. This is a fantastic resource. Do one of the given exercises each day in the gym. Remember to enforce good posture through each exercise. Which leads into...

Outside for the gym work on your posture. Check out troublesome's posture posts (they should be in the FAQ) and do the supplemental exercises while you watch TV or something. It will get you up and moving more anyway. Remember, form is the dynamic equivalent of posture. If you're posture sucks it's not going to be any better under some weight. Try to stay conscious of your posture at all times. You need to ingrain new motor patterns and it's a tough slog to erase a lifetime of bad ones.

Oh wow. I think I've rambled on enough here.