Using pointer to array in unsafe C#

In C, I can define a pointer to an array like this:

char b1[SOME_SIZE];
char (*b3)[3]=(char(*)[3])b1;

so that b3[i][j] == b1[i*3+j].

Can I declare such a pointer, b3, in unsafe C#?

My intention is to access bitmap channels as:

///...
unsafe {
    //...
    byte *target; //8bpp
    byte (*source)[3]; //24bpp
    //...
    target[x]=(source[x][0]+source[x][1]+source[x][2])/3;
    //...

I hope this way, using source[x][ch] instead of source[x*3+ch] to get some compiler optimization.