Can an Austrian pine tree be watered too much? I bought these 3 weeks ago and have watered every day thoroughly, now the green needles are falling off. I see no signs of pests.

This is usually a very hardy little tree! Are they planted in full sun? May I ask what zone you are located in?

The only thing I can think of is Dothistroma pini. This is a fungus that attacks Austrian pine of all ages, causing significant foliage loss. Check out this website; near the bottom of it is shows a picture of what this fungus looks like! Dothistroma Needle Blight of Pines - FIDL