Relevant text: Your attack gets plus x damage. X equals the total number of resource symbols on cards in your card pool that match your chars resource symbols.

Having some confusion understanding the effect. Is it a max of 3 damage or can it be more?

For instance i'm Zi Mei and have the following in my pool: Reverse flayer (2) Midnight launcher (3) Knight breaker(3) is x in this situation equal to 8?

Correctamundo. Damage pump equals total number of symbols that match on each card, including those that overlap on multiple cards.

Example: Menuett Dance + Tira = + 12 damage, fully multiplied.

Stamp for MegaGeese. I'm more partial to it with Double Grounder Beta myself

Surely even if a symbol was on every card in your card pool, and matched your character, it is still just that 1 symbol, and as such should only be counted as such?

No. It says Total number of symbols that match in your cardpool. If you had two copies of the same card in your card pool, you still have 2 cards in your cardpool.

If you have 3 cards, then you have (usually) 9 total symbols in your cardpool.

Otherwise for 3,4dmg tops it would be weak. As is its a finisher E.

Compare it with the text on The Art of Self-Defence which says "the total number of different resource symbols in your opponent's card pool...". If it meant different symbols then it would say so.