Try to reinstall stock firmware usung odin

Can you enter recovery mode by pressing button home + vol up + power at the same time and hold them until show some screen?

doesnt matter if you root or not, samsung 1st step in checking malfunction is flash stock rom.
been there twice with two rooted phone, the 1st thing they ask, do you backup your files. As we will factory reset it.

what is the problem wit ur phone?

if it's the problem of flashing.. and since still can go into download mode.. y don't u just flash the rom again and see? shouldn't send it to the sc at the first place if it's your own flashing problem.. flashing rom really voided the warranty..

try the one near federal highway. they provide better service i assume. i went there on saturday morning at 9am, i was the 2nd customer. the guy asked whether i needed the phone on the same day, i said yes. he then tried to squeeze time to repair my phone and i got it back on the same day at 1pm (they work half day on saturdays).

also i had no prove on my problem as it happens occasionally, but still he fixed it for me on the spot. very impressed with their service and attitude.
I
head to that SC and try ur luck, i think it should be fine.

P/S: i unrooted my phone and used triangle away to reset the binary counter. however i have a tonne loads of Root apps on my phone they didn't seem to bother though.

how long would they take to fix your phone tho if they were to change your motherboard?

First..see if you phone can go into download mode or not..?
- power off your phone.
- use three buttons combo (vol down +  power + home button)
If can..then flash any official firmware.
If cannot..then try use usb jig (dont know whether can be use on S3 or not) to force your phone into download mode.

If still cannot after you try all this then had to bring to service centre edi.

it shouldn't take so long.

www,sammobile.com.

search for your device and download

kinda pity seeing you 

where you staying man? PJ? if you can come down to shah alam or we can meet somewhere in the middle, i sure help you. as long as the download mode is still there, you are saved,

did you tell them you are flashing? if yes, then that why they provoke you say warranty void,

is that an official SC service center? are u able to contact them directly?
as i know, i wanted to contact the service center on Federal Highway, the only way is to call to the call center and they will check my phone's status and see whether it is up for collection. i couldnt contact the SC directly

hah, how the hell they can say that if they cant even turn on the phone? that must mean they turn it on, do you have the phone in your hand right now? or is it in their hand? grab it back first, and then try to make it go to download mode, volume down + home + power button, if can get into that mode, everything going to be alright,

if you cant go into that mode, then lets take it to the federal highway SC, lets not go to these shopping mall shop, i couldnt tell if they really samsung SC or just shop with samsung letters

no wonder
its a sep shop
take back ur phone first
the other way around