score:5
Heathrow have airside transfer buses between terminals.
You can go their website, select "Connections" and enter your details (date, incoming and outward flights) and they'll give you your itinerary.
Using tomorrow, BA142 from DEL and BA1526 to PHX, the results are:
T5 -> T3
2 hr 25 min stopover
You need 90 min to transfer from Terminal 5 to Terminal 3
Scrolling down, the steps are:
Welcome to Heathrow
Simply follow the purple signs for Flight Connections when you leave the plane.
Hop on a bus
A dedicated bus will transfer you between terminals free of charge, every 6-10 minutes.
Security check ahead
Safety first. You and your hand baggage need to pass security, so make sure any liquids you’re carrying meet the maximum 100ml rule.
Airline desks
If you need any help from your airline or handling agent, you’ll find them along your way.
Get ready for take-off
When you arrive at your departure lounge, the screens will show when your flight is ready for boarding. In the meantime, just relax.
No passport control involved.
To find out if you need a visa, use the UK government's check if you need a UK visa page. The results in your case are:
You’ll need a visa to pass through the UK in transit (unless you’re exempt)
You should apply for a Direct Airside Transit visa if you arrive in the UK on a flight and leave again without passing through immigration control.
Exemptions
You do not need a visa if you have one of the following:
- a visa for Canada, New Zealand, Australia or the USA (this can be used for travel to any country) (...)
So, no, you don't need a visa.
Unless you incoming flight is very late, you shouldn't have any issues making the connection in 2h25.
If your incoming flight is so late you can't make the connection, BA need to take care of you, and rebook you on the next available flight. There's another flight later in the day so that should do. If it's full as well they may reroute you through a different itinerary, or in the worst case they'll have to find an hotel for you (and pay for it). In that case you'll go through passport control, but even if that case, travelling to the US with a valid US visa exempts you from needing a UK visa.