Comes from the past but it won't last
It might shine first but it will rust
Ideally, there are three friends he can trust
They keep him busy, so it won't gather dust
What is it?
Hint:
Title
The second stanza as the second hint:
It both stays and moves
If it obeys the rules
A genius baby proves
When he gently pulls
How about
Newton's Cradle
Got the balls to answer?
It's made of a series of balls suspended from a frame...
Comes from the past but it won't last
It might shine first but it will rust
Early versions were made in the XVII Century. They're often made of some sort of metal
Ideally, there are three friends he can trust
Newton's three laws of motion.
They keep him busy, so it won't gather dust
After starting one, they go one for a while
It both stays and moves
If it obeys the rules
The end balls fly out while all the others are stationary, while obeying the rules of momentum
A genius baby proves
Genius=Newton, Baby->Cradle
When he gently pulls
How you start the cradle, by pulling back and releasing one of the balls
Sounds to me like
The balls outside a pawnbroker's shop. They always come in 3's, they have a long tradition, they do start shiny and will rust since they hang outside. I suppose it might be busy inside a pawnbroker's but this is one of the weak parts of my argument.
This doesn't seem to make too much sense (as it doesn't fit the second hint), but I will give it a shot anyways...
The answer is:
Home plate in baseball
It comes from the past but won't last because
The home plate is the place where you started at first (past), but also where you could potentially end up.
It will shine at first it will rust because
Obviously it will get "rust" eventually
And finally his three friends are
The other three bases, which helps the home plate to prevent the other team from scoring, or getting to the home plate.
I know it's already been answered, but I wanted to include my answer that I think works moderately well before including the second stanza.
Answer:
A clock
Comes from the past but it won't last
Wall clocks have been around for centuries. Nicer clocks often become antique items and/or are considered family heirlooms and passed down for many years but will eventually wear out and cease functioning.
It might shine first but it will rust
Without proper maintenance, the mechanisms in a clock can gather moisture, rust, and seize up.
Ideally, there are three friends he can trust
They keep him busy, so it won't gather dust
Also related to the title -- wall clocks are often weight-driven, the larger ones with three weights. While traditionally cylindrical, ball weights also exist. The slow descent of the weights can keep a clock running for several days at a time.
It still (kind of almost) works with the second stanza, with the exception of a pretty specific line.
It both stays and moves
If it obeys the rules
The pendulum and weights keep the gears moving, but the clock stays in place.
A genius baby proves
Yeah, I got nothing for this one.
When he gently pulls
To wind the clock, you pull the chain for each weight.
