Hi i'm making a fighter subclass with homebrew spells, and i can't find a way to automatically add the higher level spells.

For instance i have a 3rd level spell that scales 1d8 with each spell level. eventhough my level is high enough the spell levels above level 3 are not showing in my spell list.
I can manually add the spell 7 more times all the way up to level 9 but then i have the problem of having 7 different attacks of said spell under the actions tab.

I'm quite new at this and been searching on the forum for a solution but wasn't able to find one

One can only upcast spells like that if they spend Spell Slots. If there’s no spell slots to spend, then it won’t display higher level castings of the spells because the rules disallow it.

Is there a way to add spell slots?

You can make it a ⅓ caster with the exact same spell progression as the Eldritch Knight has. Please see the Homebrew Subclasses & Species FAQ #7 for a link to instructions on how to do that: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq). You might also find other helpful info in that FAQ.