The MINIMAL Force !!!
Okay, there's all this about finding everything possible and having every character too...

But what if you decide you DON'T want every fighter possible ???

There is a way to have a minimal Force.  Your entire party could be made of only 10 fighters instead of the maximum 19 (by the end of the game - including Fynnding).

The MINIMAL Force would be made of:

vs. Arrawnt:  Synbios, Dantares, Masqurin, Grace, Obright, Cybel, Eldar, Kahn and Murasame.
vs. GigaBreaker (Dam):  Fynnding, plus hired merc(s).

Fynnding switches to second party automatically, so you can't have the problem of having zero fighters at the Dam.  You can hire the 4 mercs from the Inn to better your chances at the Dam, but Fynnding alone could do it (see how below).
How to get the MINIMAL Force ?
Don't talk to Hayward and let him die at the Saraband Pier before he joins the Force.
Don't talk to Irene and let her die at the Railhead Storeroom before she joins the Force.
Don't talk to Noon after defeating the Vandal, or kill him as Kyon-Shi (if you know which one is Noon).
Don't talk to Justin in Vagabond 1.
Don't talk to Horst in Aspia .
Penn: Don't buy egg/don't take Chicken Feed/don't feed chicken/don't hatch/don't talk to him in Storich.
Ratchet: Don't activate switch in Mansion/don't talk to him in Vagabond.
Don't push Frank into Headquarters.
Hagane: Don't inspect the pillar in Tower Ruins.
ALL the pictures on this page were taken from the Shining Force III Picture Gallery, from RPG-O-Mania.  Pictures of every character from ALL 3 scenarios are available there.  A must-see !  Pictures used with permission from Doc Ower.
How can Fynnding do it alone ?
Even an untrained Fynnding can defeat the GigaBreaker at the Dam all by himself.  You simply need to follow these simple steps (to simplify, I will suppose the Dam is at the NORTH.:

1- Equip him with a Life Ring and Silk Wing;
2- Give him the Ibiru Ring - DON'T EQUIP;
3- Cross the chasm in a straight line "westward";
4- Reach the imaginary edge (western edge of western cliff);
5- Head towards GigaBreaker by staying on the edge;
6- Position yourself 2 squares X 2 squares northwest of GigaBreaker; **
7- Save the game;
8- Use Ibiru Ring on GigaBreaker (if Ring cracks, reset the game);
9- If Griffon attacks, defeat it before returning to Gigabreaker;
10- Save before each use of Ibiru Ring.  Use until victory.

** Positioning yourself anywhere but at the mentioned position will either make you subject to a magic attack by the Sorcerer or you will be too far to attack from a distance.  Just do as I say, 'kay ?