In the base 3pool, your risks are as follow:
Smart contract issues with Iron Finance: While audits and bug bounties can significantly mitigate risk, there will always remain an inherent risk with all smart contracts in DeFi.
Systemic (macro and general, market-wide) issues with the stable coins in the pool: If one of the stablecoins in the pool goes significantly down below the peg of $1 and never returns to the peg, it'll effectively mean that pool liquidity providers hold almost all their liquidity in that currency.
In the metapools (3pool-PUSD, 3pool-xUSD, and any other metapool), your risks are as follow:
Smart contract issues with Iron Finance
Systemic issues with the stable coins in those pools
Systemic and smart contract issues with PUSD, xUSD and any other metapool stablecoin