Exhusband gave 3 year old (34 lbs)childrens Robitussin (guaifenesin) for common cold symptoms. She has never had it before and he didnt speak to doc. Concerns??

Not a problem... It is one of the few meds I would use in such kids & has a five + decade track record of safety when used in proper doses.
How much did he give. We generally don't give this medicine this young - you are correct. You don't say how much or which formula specifically he gave. If it was a small amount it should be ok. Just watch carefully for next few hours for things like extreme agitation or restlessness. .