Hi Henry,
1] Blackfin SPI module do not supprt 3-wire protocol. There are separate MISO and MOSI lines for data transfer.
But I guess, you can try to implement some part of it by connecting MISO & MOSI line together and disabling unused pin in PORT muxing. (disabling pin in PORT muxing means, configuring it in GP input mode with input buffer disabled such that it will tri-state).
If BF SPI is master: Whenever you want to receive, enable MISO pin in PORT mux and disabled MOSI. While tramsmitting, enable MOSI pin.
If BF SPI is slave: You can implement receiver, keep MOSI pin disabled. Transmitter would be tricky to implement (as you may get some commands from master first).
2] How many dma channels can run at the same time? We will use SPI, PPI, UART, I2S at the same time.
>> It depends on request rates of these peripherals DMAs. Note that, there will arbitration between active DMA requests. The DAB&DCB bus is 16-bit wide and it operates at SCLK rate.
Refer chip bus hierarchy chapter for more details.
-Prashant