I'm new and also having this issue - I've been using the lesson that Bill suggested - it's called "Spider Legs Warm-Up Exercise'.

I do this for 5 minutes at the beginning of each lesson. I've been doing this for a week and I've found that I can keep all three fingers down for the first few strings. I'm still working on keeping all my fingers down at once on strings 4-6 but I'm gradually getting there.

I hope you find a strategy/exercise that works for you.

Lyndall