Yes that's plenty of time. Gate to gate it can be done in 45 minutes or so, depending on various factors, so you should be fine even if an hour late. If it is one ticket then BA will rebook you on to the next service, and though it may look full now it may not be so on the day. Just follow the purple signs for Flight Connections to Terminal 3, it's a bus that goes over from airside, it takes 12 minutes and runs every 6 minutes. Your route is very clearly marked, just look for those purple signs and you shouldn't have a problem.

The only thing to watch is that your flight from Brazil may well end up at a B gate, so terminal 5B - you will know this when you get off since there are white illuminated panels. More than 50% chance of this, I would say. In which case the bus for T3 also picks up directly from that terminal and you don't need to follow the herd to the main building, it is particularly stress free from there. There is a guide to this in the Dashboard.