Judah, thanks for your reply. After I went through the running CSL and BIOS programs once again, I've finally noticed the key difference:
IRQ_enable(IRQ_EVT_INT3); // CSL C55_enableInt(C55_INT3); // DSP/BIOS
are not the same, though looking identical. I'm really wondering, why DSP/BIOS defines C55_INT3 differently comparing to CSL, namely to be not INT3 actually, but the BIT 3 in IER0, while IRQ_EVT_INT3 is defined as 11 (staying indeed for INT3). Analogously with a HWI declaration: HWI_INT3 doesn't stay for INT3, but HWI_INT11 would be right.