Lets do some quick math here!
Here are the drop rates:
Torment I: 25%
Torment II: 28%
Torment III: 33%
Torment IV: 38%
Torment V: 43%
Torment VI: 50%
Assuming you create three games and pop three portals at once to have a go at all bosses, at T3, you have a fixed three tries on all of the bosses.
To get a part from all bosses in one round of portals is 0.33 * 0.33 * 0.33, coming down to around 3,5%, which is not a lot - I would not bother with just three machines.
If we conclude that there is a 2/3 (around 66%) chance to NOT get a part from a boss, then if you kill a boss two times, there is now a 4/9 (44%) to NOT get a part. Again, for the third time, the chances are 8/27 (33%) chance to NOT get a single drop from a single boss.
This translates to around 67% chance to get a drop from a specific boss in three runs.
Now, if you want drops from all bosses, it's 0.67 * 0.67 * 0.67, which comes to around 30% to get at least a single drop from all of the bosses.
For comparison, if we were to kill the bosses only two times, this chance a measly 8,5%. Alternatively, killing them four times would net a ~52% chance.
So, are you feeling lucky?