Use all you got
It is ok to train with the prices per day to predict 3 days in advance.
Suppose you have the data:
[ 1, 20, 33, 4444, 2, 21, 34, 4445, 3, 22, 35, 4446]
t-11 t-10 t-9 t-8 t-7 t-6 t-5 t-4 t-3 t-2 t-1 t
and you want to predict
t+3 (spoiler: should be
If you look backward in steps of
3 you will be looking at
[t, t-3, t-6, t-9] which corresponds to
[4446, 3, 21, 33]. Obviously, it will be much harder to predict
36 from that, compared to if we had looked at
[t-1, t-5, t-9] = [35, 34, 33].
Therefore, it is encouraged, to feed all the data you have to your model.
If you are using a neural network, your network should be able to learn this patterns for you.
If you are using some other type of regressor, you might need to figure out which kernel to use your self.