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 a power2max or Sigeyi SRAM MTB Power Meter Spider
Another option is to go with either the power2max SRAM MTB Power Meter Spider or Sigeyi AXO SRAM MTB Power Meter Spider. The power2max and Sigeyi are very similar spider-based MTB power meters, however they fit directly onto the 3-bolt SRAM crankset you have. This makes them much more affordable options as you don’t have to buy a new crankset. Both power2max and Sigeyi make wonderful power meters. We use them on several of our own bikes. They measure power from both legs, are reliable, accurate and are reasonably priced.
Please note, both of these power meters require a 104 BCD 4-bolt power2max or Sigeyi MTB chainring.