This is very easily done. The only possible thing to watch out for is that only the account holder (your wife) or an authorized user (this isn't necessarily any member of the family) may make changes to the account(s). It's very easy for your wife to add you as an authorized user if you aren't one already (she just has to login online and there is a "my profile" tab. Scroll down to "Authorized User-Retalil Store")
You can upgrade any line which qualifies at the time. For example, for our family plan all four lines became eligible recently but only I have upgraded so far. My line thus becomes eligible sooner than the rest of my family's because it was the first to be upgraded.
In essence you get a staggered schedule for your upgrades. If this concerns you then try to have all three lines upgraded around the same time. If it really doesn't make a difference to you, then no need to sweat it. Each line retains its own upgrade schedule depending on what was bought when.