At this point, it's just a matter of pairing up which GPIOs you can export the interface to, versus which GPIOs are being brought out to the 70-pin headers on the Gumstix. Unfortunately, it appears that gpio178-182 are not brought out to the 70-pin headers on the Gumstix, and since those are the only GPIOs that you can mux MCSPI2 out on, you can't use it.
However, you can bring mcspi1_cs2 out on gpio176, which should let you control three SPI devices on MCSPI1 (cs0, cs1, and cs2). cs3 (gpio177) isn't available though.
The GPIOs that are brought out on the Gumstix are labeled as such.
If you just do a search in the TRM for "mcspi3" you can see where it can be brought out
mcspi3_clk - gpio88 or gpio130 or gpio17
mcspi3_simo - gpio89 or gpio131 or gpio14
mcspi3_somi - gpio90 or gpio132 or gpio15
mcspi3_cs0 - gpio91 or gpio135 or gpio16
mcspi3_cs1 - gpio92 or gpio134 or gpio21
Yes, from the Alcatraz schematics. I'm sure board real estate has a lot to do with it, there's just not enough room to bring out every single GPIO. Also, many of those pins are used internally for MMC, wifi, bluetooth, etc, so even if they were brought out to the 70 pin headers you wouldn't be able to use them anyway.