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. However, 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. Therefore, there is no need to change your crankset. power2max makes wonderful power meters. We use them on a number 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.