I just planted three plants and I am not sure how much water they need.

A newly planted tree should be watered every day for the first 3-4 weeks. Then gradually cut back. Typically on a drip system about 2-3 times per week about 4-5 gallons of water.
Of course this may vary depending on the type of tree you purchased.

Most large trees will eventually find water on their own. This is when they become well established.

Be careful with fertilizing right after planting...could burn root system (rare but does happen). Wait at least 2 weeks for fertilizing then fertilize with a good tree and shrub fertilizer.

here's the deal.... to do well, your trees need to send out new little 'feeder roots'... they're very small, almost hairy.... if the soil is dry, they wither up.... if the soil is moist, they can push thru more easily and take up what the tree needs at the same time.... so... your job is to keep the soil MOIST so those little roots can do their thing.... best way to know if it needs water or not is to open a hole with a hand trowel near where the root ball is sitting in the hole.... put your fingers in the hole you''ve made and test the soil for moisture!!!... if it's moist to your touch, you can try again tomorrow... if it's dry, then water today!!.... if it's soggy, it rained last night!!!.... *smile*..... use a bucket about the same size as the pot the tree came in... fill it with water and slowly pour that water around the area of the root ball and out a little , too, to encourage the roots to go hunting for more .....(be sure to close the trowel -hole back up, too!)..... if you get good at this, in a few weeks you'll be able to use another bucket, put smallish holes around the side/bottom so that water just sorta seeps out.....fill the bucket and let it sit beside the tree until empty...... as long as your soil is NOT frozen soil, you should continue to water your trees..... stop only after the soil is so cold that water doesn't sink down.... after that, it's up to the rains and snows until the temperatures warm up... but when they do, be sure to go check the tree's ...... see, underground, there's still root growing going on, even when the tree on top looks totally dormant!.... and if it warms up enuff to melt snow, those roots are still busy!....

newly planted trees need attention for the first year... lots of attention for the first six or eight weeks, at least... then weekly after that to check on their progress.... we transplant them, so we need to care for them after we do!!!...

the only thing to be careful of is that the trees do not sit IN water.... the soil must drain off water, too... drowning can happen to trees, too.....

Generally speaking, newly planted trees need a bit more for the first week than do older ones. But again, this depends upon what ytpe of soil you have, and what type of trees you have planted.

I do not know where you live nor the types of trees that you've planted so I suggest that you call a local nursery for accurate, specific advice.

2 times daily you cannot overdue, what it doesn't need will go into the soil, follow this for at least 4 weeks if you live in a hot country for 2 month, don't forget some natural fertilizer