I'm not going to say a weight to start with in absolute terms. I could try to suggest something as a percentage of your one rep max, but I think it's better to start training with something and adjust as required.
The Greyskull LP Program combines 2x5 with 1x5+ (as many as possible with good form). So start with something you're comfortable doing 3x5. For example, If 85kg is your 5x3, it's going to be a bit less weight than that, I'd say no more than 80kg. Whatever you pick, make sure it's something you know you can manage because failing on the first workout sucks (for consistency and motivation) and because the program provides a mechanism for increasing weights faster.
You can double up your weight increases. Usually you'll be putting on 2.5lb/1.25kg increments, but, if you're so comfortable with the current weight that you're getting 10-12+ reps in that third set, you just double that increase for next time (5lb/2.5kg).