I found for a random date in May 2017 on Amex Travel, its $1425 per person return in Economy via Thai Airways. In raw Amex Points, its 142544 pts. Usually not the greatest value for your points to redeem with raw Amex pts (1 cent per point).
You could do points +cash if booking via Amex Travel. For your 90k amex points, you save about $900. So you would have to pay $500 plus and 90k amex pts for 1 return ticket.
If you have decided to fly on Thai, you could transfer your Amex pts to Krisflyer and do a Star Alliance redemption for Thai Airways for 100k Krisflyer. However, there may be a hefty surcharge/tax via Krisflyer. For Star Alliance redemptions, you have to ring Krisflyer.
You could always top up MR for 2.5 cents per point if very desperate.
AFAIK, I don’t think it is necessary to book all 3 person in a single reservation. If seat allocation is a concern, you could pre-select seats during or right after booking (even if on 3 individual reservations). There are pros and cons to having all 3 on a single itinerary or separate.
Sometimes it may be possible to ring up later and combine itineraries too.
So if you are happy to book 1 ticket via Amex Travel, you could book the other 2 via Thai Airways or sites like skyscanner, which could be cheaper than Amex Travel.
Some tips in case you haven’t considered:
Check other airports near London, or other airports in Europe. There may be deals going on and you could take a cheap intra-Europe flight to London.
Flights departing London has hefty ‘departure tax’, about $400 from memory. You could avoid it by flying out from Europe via Paris or other airports.
Worthwhile checking costs for one way booking/redemptions, usually they don’t cost all that different to returns and give you a lot more flexibility.
Good luck and please do ask if you have more questions.