I'm trying to convert a work trip from Newark to London into a mileage run by routing back via San Francisco. If I price that up on united.com as a EWR-LHR-SFO multi-city return and a separate SFO-EWR one-way the total comes to $800 in Y. But If I try and book it as a three-leg multi-city trip it prices at $5100 (ouch).

Anyone know if there a way to book all three legs as single trip to reduce irrop/misconnection risk? Or do I just have to choose a long layover in SFO and hope for the best?