You have tried to 1) Remove you're battery 2) Re-insert it and press Vol+, Vol-, Home button & power-on button at the same time?
And if this don't work... I heard of a method with a Usb plug to access download mode on some phones... but you'll have to buy a plug, it's about 4$ on ebay
Samsung Galaxy 3 [i5800 - Froyo 2.2
Samsung Galaxy Ace 832Mhz
[S5830i - Gingerbread 2.3.6
White Samsung Galaxy S3 LTE 16Go QuadCore 1.4Ghz - 2GB Ram
[GT-I9305 - CM11/KK - 4.4.4
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