I'm a completionist without much time on my hands. Do I need to make 3 characters to see all of the game, or will being a thief with magic and parry let me do that? Can I use one character to join all 3 guilds and follow all 3 special path-options, become a paladin, etc. or do I really need to make 3 to see everything?

I believe that is sufficient for experiencing the entirety of the first game, but beyond that, I am unsure. I seem to recall that the EOF sidequest in QFGII could only be completed by a fighter, but don't quote me on that. Ad for later games in the series, I am pretty sure that there will be certain portions III-V that you will be unable to experience without being in a certain class.

Going with a Fighter->Paladin (no thief skills) and a Magic User->Wizard (who also has thief skills) would cover [i]
most[/i] of the series content in two play-throughs, but you'd still miss out on the thief-class-specific portions of QFG3, and couldn't do all three endings of QFG2 (unless you play the end sequence multiple times with the mage/thief).

With a single character? An all-skills paladin would be able to see a reasonable amount of the game but would miss out on the thief-specific content of #2 and #3, and could not access most of the wizard-related material in #3, #4, and #5. He would likewise only be able to access certain end sequences in #2, and undertaking the thief missions in #5 would prove awkward due to the Honor hit.

Likewise, certain marriage options in #5 would be blocked off with either of these approaches.

Three play throughs (as Fighter->Paladin, Magic User->Wizard, and Thief) without cross-class skills (to be on the safe side) are necessary to see everything. (There is only one thing that the Fighter can do and the Paladin can't, which is marry a certain character in QFG5 - but the Wizard can marry that one as long as he doesn't have the Lock Picking skill.)

2 Things actually, you forgot the "Become a Paladin" ceremony on QfG3, if you play it with a Fighter. Ending sequence in 3 is also a bit different for fighters and paladins, but as I recall it's just what item you need to use.