Oh and you should have some time for shopping on the outbound flight if you're willing to sacrifice lounge time, T3 has some great shopping choices, albeit with a very premium skew (think 5th Avenue / Bond Street-type designer shops mostly), unfortunately you won't have access to the T5 shops on the outbound flight (there's a way to request to be escorted to shop in a different terminal at LHR, but I'd think with a three hour layover it possibly isn't worth the hassle).