Constraining non--BPS interactions from counterterms in three loop maximal supergravity

The structure of one, two and three loop counterterms imposes strong constraints on several non--BPS interactions in the low momentum expansion of the three loop four graviton amplitude in maximal supergravity. The constraints are imposed by demanding consistency with string amplitudes. We analyze these constraints imposed on the D^8 R^4 interaction in 11 dimensional supergravity compactified on T^2. We also discuss partial contributions from counterterms to interactions at higher orders in the momentum expansion.