My phone froze and couldn't turn on back?

Hello,

My Z1 Compact froze once, restarted and showed Service sign. After a while Phone turned on. 2 Days later Phone battery drained at night. I turned on device morning and used during the charge. After a few minutes phone froze and couldn’t turn on. Restart options work but when clicking power button red led blinking three times.

I have sent to repairer. They say, motherboard dead. I couldn’t check phone right now but as they said phone open and turn off again. I couldn’t believe problem is motherboard.

May you help me about this?