How to check balance of a wallet via third-party API?

I'm a developer.

I've already added payments in Solana and ETH onto my website and now want to add ones in Monero.

So I'll generate a 100 wallets that will be assigned to each user. How will I checking their balances? Will it be possible at all, without my own node, or with a third-party API only? Or will I have to run a node?

The key is to be able to do it automatically, in a web app.