If I owned a company that did plumbing, electrical and air I would call it EgolServices.com and I would have ONE website.
All of the links that any part of that business earned would benefit the entire domain.
And all of the trust that my plumbing staff earned would generate repeat business when the same client needed air. All of the trust that my electric staff earns would generate repeat business for my plumbers.
Why make clients make separate decisions when you are out there doing good work?
The big air conditioner that you want in the house will probably need a more substantial wiring. Just call EgolServices and they will take care of it.
I think that this is a nobrainer. NoBrainer.