The thing that you don't seem to understand is that an advanced job is not a 3rd job it's only a job that you have to unlock. Once unlocked they are like any other jobs. You can only use 2 jobs at any given time. So in your exemple, you will be summoner(lvl70)/black mage(lvl35) or black mage (lvl70)/summoner(lvl35) or summoner(lvl70)/thief(lvl35) or black mage (lvl70)/thief(lvl35) or thief(lvl35)/summoner(lvl17) or thief(lvl35)/black mage(lvl17). You can switch between those possibilities as many times as you want but you will never be a summoner(lvl70)/blackmage(lvl70)/thief(lvl35).

So if you chose to be blackmage(lvl70)/thief(lvl35) your summoner job will still be lvl 70 but you wont use any of its abilities or stats or spells.