3 knots winds are very little. Probably too little for the spi. If the chute won't fill properly (hanging limp) than you're faster without a set spi.

Best weather for a spi is 5 to 12 knots. Biggest speed increase there; a real exitment inducer. In higher winds the boost is a little less; although still significant.

Picture was taken in Australia (2005) at the Port Kembla spinnaker run event. Skipper is Gary Maskiel. He is sailing his own one-off Altered F16. Actually he revamped an Boyer mark 5 A-cat (2001) and made an F16 out of it. Hence the name "A-ltered". Needless to say that the A-cat sailors had a scolding thought or two about that !

Wouter