Welcome to AF.
The best thing to do is replace the lower intake manifold (LIM) gaskets and the plastic UIM plenum to include UIM gaskets. The UIM kits like from Dorman will have all parts needed to do that job. On the LIM gaskets you can go with the FelPro brand or the revised GM aluminum framed gasket set.
Just send me a PM with your regular email address and I will send you a guide on Series II 3800 engine UIM and LIM replacement.
Don't wait too long because you could hydrolock your engine if coolant is consumed in the intake or ruin the internal bearings if coolant is leaking in to the crankcase.