Does the seymor duncan spb-3 fit on a squier mustang bass?

Are you possibly referring to the new Classic Vibe Mustang Bass?
The SPB-3 is shaped and sized like regular P bass pickups, so I would recommend checking to see if the cavity under the pickguard can fit it. You can probably get a replacement pickguard in case your Mustang comes with the traditional Mustang split-coil pickups.
 
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it should be pretty obvious from visuals
if the pickups look like they're the same shape, it will fit
if the stock ones look smaller and rounded off, it will require at least some work
as jaxon5 mentioned, the wood under the pickguard may already be routed for the bigger pickups, if so that's a huge help