I want to do the horse trekking of 3 days and then travel to xi'an via chengdu.

Oh, the time will be tight.
You can take a train at Chengdu Railway Station in the afternoon or night. Then it's possible to arrive at Xian in the next early morning.

you know when the trek finis? because its 8 hours to Chengdu and the last train to Xi'an leave at 22:16

Well, it mainly depends on you.
Generally speaking, you lend the horse by the day. Most tourists will choose to ride the horse for one day to tour around the whole beautiful scenery.
But if you just wanna to experience the horse riding, you can return back at any time. Just settle it down with the horse owner or guide before setting out.
If you wanna to stay at there for a longer time, it's suggested to stay at county for one night.
Then go to Chengdu in the next early morning.

better to take a flight streight to Xian from Huanglong