I have a character with the hammer of thunderbolts, gauntlets of ogre str and a fire giants belt, making his score a total of 30 because of their boosts.  This is causing my dmg modifier to be 30 instead of its intended number.  Can anyone explain this to me?

Please paste a copy of the URL for that character sheet so a Mod or Staffer (not me) can look into it for you.

https://www.dndbeyond.com/profile/Bigheaded00/characters/20520694

I would see if removing your homebrew weapon does the trick. Putting the gauntlets, belt, and hammer on a test character gave me +10 STR mod and +10 or +11 to damage.

Gauntlets of Ogre Power doesn't provide a stacking bonus. It's an either/or type, similar to Headband of Intellect or the Barkskin spell.

Essentially it checks if your strength is less than 19 with all other bonuses and items considered : if it is, it sets our strength to 19, if it isn't then the item does nothing.

The Belt of Giant Strength does the same thing just with different numbers depending on type. So, again, no stacking bonus.

The Hammer of Thunderbolts provides a stacking bonus. The intent is that this will stack with the strength score providing by the other items, but the RAW is that it doesn't because of how the other items work and are worded. 

If we go by the RAI, though, this should put your score at 29 (the fire giant belt takes precedence and puts your score at 25, the +4 bonus from hammer makes it 29). This is a +9 Strength Modifier. If you're getting anything different than either one of the items is broken or you have homebrew that hasn't been set up properly. Add/remove one by one to find the culprit and once you have check with the author of the homebrew.