I need to looking for the job and I have 3 kids in age 12yrs,10yrs and 8yrs old. During school holiday I cannot go to work,if I put 3 all them to holiday care cost me a lot.I want to ask or advice about my 12yrs old and 10yrs old should stay by themselves at home

First will your work let you off to watch the kids, most will not, so holiday care is often better than losing a job.

But on the 12 and 10 year old, how mature are they, do the 12 and 10 year old fight,

Have you left the 12 and 10 alone for shorter periods, such as 1 to 4 hours for trial of how they will do.

Can you put in video camera linked to internet so you can view the inside of the house and the kids from a computer at work.

Next are they family that could watch, or is there someone near by that could just go in every couple hours and check on them, and be a "safe" haven if they had a problem.

Yeah, It would depend on how mature they are and how well they get along.
Good Sujestions would be to make sure one of them has a cell phone, so they can call you if something goes wrong, but your number on speed dial. ;) Makes it faster and easier.
Maybe one day, take the 8 year old to the store with you, and leave the other two at home. Be gone for maybe about and hour or so, to see how they do. If it's out of hand after only and hour, you might want to start looking for cheap child care. Lol But if all goes well, then maybe it won't be so bad. ;)
Heh, I'm ADHD, so leaving me home alone at that age was like my mom's suicide. Bahaha! I was a very... lost and hyper child. Lol Just thought I would throw that in there. ;) Hope I've helped at all. :D