One writer says that Jesus found a young donkey, as if he himself sought it out.
Another writer says that Jesus sent his disciples ahead with explicit instructions on how to find a specific donkey, along with her colt.
Yet another writer says they were sent with instructions on finding only a colt.
And yet another writer says that Jesus sent his disciples to fetch a single colt, but to promise that they would bring it back.
If you read the stories carefully, there are some obvious discrepancies. If you read enough other stories in the Gospels, you will find other such discrepancies—some that seem more or less significant than the donkey / colt business. Some try to consolidate these accounts. Some take it to mean that nothing in the Scriptures is reliable.
Whatever.
In most cases, I think getting hung up on discrepancies is a distraction. The similarities serve to funnel us towards the real point: Jesus rode into Jerusalem on a humble beast of burden. Not on a war horse, like the rulers of Rome. If we spend too much time and energy trying to do that math between the discrepancies, we are likely to miss the marked difference between the Kingdom of God and the Empires of Mankind.