How to perform IQ signal decomposition for 8-PSK?

Question

Signal processing

Good night, colleagues)
The introductory is as follows: I am assembling an 8-PSK modulator model in Simulink. In the modulator circuit, it is required to multiply the cosine wave and the sinusoid by the I and Q components of the signal, respectively.
In the case of QPSK, everything would be very clear: we take Buffer, we make packs of two bits, in each pack we take the first bit for I, the second for Q, while converting 0 to -1.
But what if I have 8-PSK and packs, respectively, already consist of three bits?