Southeast Asia forum

Many places have a pack-closet where they keep guest's bags for just these circumstances -as lizzy says, it is a good idea to lock your bag up if you keep valuables in it - though that is why i recommend not traveling with any valuables - the most valuable thing I usually have is my pack. I've left my bag a number of times but was only afraid it'd be stolen because then I'd come back and have nothing clean to wear.

If your place doesn't have a storage spot - then if you've booked your 3 day trek with a tour-spot, they should certainly have same type closet for just such occasion. Chances are these are very safe spots to store things, as the last thing these businesses want is negative press about letting things get stolen.

I run a guesthouse in Chiang Mai (and trekked around here in the good ol days) and yes you can safely store your big bag at the guesthouse while you go trekking. I have never heard of theft occurring in this situation and usually your bag is going to be stored in a secure place that only the staff have access. But it goes without saying don't store anything of high value. Either keep it with you during the trek or ensure that the guesthouse stores it securely (not in the general bag area/closet). All this assumes you booked the trek through the guesthouse itself. At least with the cheaper places if you book the trek elsewhere they might not store the bag for you.