The Quarq SRAM XX1 Eagle DUB Power Meter works with SRAM XX1 Eagle DUB 8-bolt cranksets. Since you have the 3-bolt interface, this means your SRAM crankset is not compatible with the XX1 Power Meter. Therefore, you do have two options.
Option #1: Buy the SRAM XX1 Power Meter + New 8-Bolt Cranks
Option 1 is to purchase the Quarq SRAM XX1 Eagle DUB Power Meter with the 8-bolt cranks that will fit the power meter (these items can be bundled together on our product page). This way, you will have a compatible crankset for the power meter spider. Please note, since the 8-bolt cranks use the DUB interface, you would need to install a SRAM DUB MTB bottom bracket on your bike if you don’t already have one.
Option #2: Buy the power2max SRAM MTB Power Meter Spider
Another option is to go with the power2max SRAM MTB Power Meter Spider. The power2max is a very similar spider-based MTB power meter, however it fits directly on to the 3-bolt SRAM crankset you have. This makes it a much more affordable option as you don’t have to buy a new crankset. power2max makes wonderful power meters. We use them on several of our own bikes. They measure power from both legs, are super reliable, accurate and are reasonably priced.
Please note, both of these power meters require a 104 BCD 4-bolt MTB chainring.