Don't get a triple GRP unless you absolutely need it - they weigh an absolute tonne!! A triple 3.6m aluminium ladder will probably do - anything higher and I would consider alternative access methods.
Double would be easier if you can. I have a triple aluminium ladder and it's bloody heavy and a nightmare to move while extended compared to the double
Fibreglass are probably better. However they are a ridiculous price for a ladder that size so if it was me I'd stick with aluminiumThe lads in work use triples but a double would probably be okay for what I need it for really. Any recommendations on brands or whether to go for aluminium or fibreglass?
Zarges skymaster are the most popular a frame/triple extension ladder in the entertainment industry, they are quite chunky but are very stable and easy to work on for longer periods.