He fears they may attack the floating station with his gf and a bunch of innocent people on it if he resists or even the collateral damage from their fight might affect it so he doesn’t want to risk it, and…
And this is the real important part, as strong as Allen is, it’s made quite clear he’s no match for 1 Viltrumite fighter, nevermind three. It’s stated in the show that as strong as he was made, he can’t even beat 1 Viltrumite, at least of all the ones he’s fought in the past so it stands to be that he would have no chance at beating three at once. Maybe if he was fighting a lower ranked one by themselves he’d stand a chance, but no way vs three.
Probably a couple reasons.
He fears they may attack the floating station with his gf and a bunch of innocent people on it if he resists or even the collateral damage from their fight might affect it so he doesn’t want to risk it, and…
And this is the real important part, as strong as Allen is, it’s made quite clear he’s no match for 1 Viltrumite fighter, nevermind three. It’s stated in the show that as strong as he was made, he can’t even beat 1 Viltrumite, at least of all the ones he’s fought in the past so it stands to be that he would have no chance at beating three at once. Maybe if he was fighting a lower ranked one by themselves he’d stand a chance, but no way vs three.