How do you think of 3 bar charge moves?

For attacking, bad.

For defending, good.

However, some are at least OK when attacking. For example, the two you listed, while certainly not ideal, are two of the best 3 bar moves. They both do decent dmg while having a very short cooldown.

For Magneton, Discharge is his only electric charge move and his strong typing makes him usable with it. For Gyarados, specifically, it's fast enough that you can dodge every Dragon Pulse after you use it.

For Exeggutor, ZH/Seed is actually a fine moveset for attacking. Again, it's not the best, but at least you'll be able to dodge the charge moves. He has strong enough stats that I wouldn't be afraid to use seed bomb until I got a better one with Solar Beam

Seed bomb is good for defense for Exeggutor.

Generally 3 charge moves are for defense.

I'd say for attacking, they are mostly weaker than 2-bar charge moves. However, in some matchups they can be more fun to use than 1-bar moves. Examples:

Discharge/Thunder Punch: Perfect for Gyarados with Twister
Seed Bomb: Perfect for Vaporeon with Water Pulse
X-Scissor: Perfect for Exeggutor with Confusion

Only real useful 3-bar charge move is X-Scissor...

Fast to use for all the bugs who have very few HP to begin with and supereffective vs. Exeggutor and Alakazam.

Depends of the pokemon. In jolteon, for example, discharge is bad, but magneton's best charge attacks are discharge and flash cannon, and they work ok. In the bugs case the best charge move is megahorn, but none bug mon can learn it, so the best is bug buzz, for the ones that can learn it, but the best bug in game still pinsir with X-scizor, it is all a matter of perspective, but I am sure that pinsir would be better if he had megahorn.

Omastar with Rock Slide is good for defense and attack. Other than that, seed bomb is the only other I really like.