Python 2, 99 bytes

from random import*
n=input()
while 1:b=sample([a*-~a/2for a in range(n+1)]*3,3);n-sum(b)or exit(b)

Try it online!

I'm pretty amazed this is shorter than itertools or a triple list comprehension! It (eventually) spits out a random answer every time you run it.

Python 2, 99 bytes

from random import*
n=input()
while 1:b=sample([a*-~a/2for a in range(n+1)]*3,3);n-sum(b)or exit(b)

Try it online!

I'm pretty amazed this is shorter than itertools or a triple list comprehension! It (eventually) spits out a random answer every time you run it.

Two 102s:

n=input();r=[a*-~a/2for a in range(n+1)];print[(a,b,c)for a in r for b in r for c in r if a+b+c==n][0]
def f(n):r=[a*-~a/2for a in range(n+1)];return[(a,b,c)for a in r for b in r for c in r if a+b+c==n][0]

itertools looks to be 106:

from itertools import*;lambda n:[x for x in product([a*-~a/2for a in range(n+1)],repeat=3)if sum(x)==n][0]

Python 2, 99 bytes

from random import*
n=input()
while 1:b=sample([a*-~a/2for a in range(n+1)]*3,3);n-sum(b)or exit(b)

Try it online!

Python 2, 99 bytes

from random import*
n=input()
while 1:b=sample([a*-~a/2for a in range(n+1)]*3,3);n-sum(b)or exit(b)

Try it online!

I'm pretty amazed this is shorter than itertools or a triple list comprehension! It (eventually) spits out a random answer every time you run it.

Python 2, 102 bytes

n=input()
r=[a*-~a/2for a in range(n+1)]
print[(a,b,c)for a in r for b in r for c in r if a+b+c==n][0]

Try it online!

Python 2, 99 bytes

from random import*
n=input()
while 1:b=sample([a*-~a/2for a in range(n+1)]*3,3);n-sum(b)or exit(b)

Try it online!