I contacted every single Hyatt hotel in London for my July stay for a family of 4 (two young kids). To my surprise (and dismay) every single property stated that their capacity rules regarding our family of 4 was exactly as shown online. I would estimate that over the past 5 years, more than half of the roughly 50-60 properties I have contacted regarding this same question have given us options beyond what was shown on the website. Anyway, I explain this all to say that there's a good chance that the London hotels follow precisely what the website indicates when you do a search for your family of 3. Ymmv since yours is 3 and not 4, but that's what happened for me recently.