Your only choice is optical as the USB port on the AE will not support audio. The jitter specs on the AE are not great but Bel Canto claims that the DAC 3 has excellent jitter rejection so this may not be a factor. Many will claim a glass cable is better than plastic so you may want to try that comparison even though it seems to me that if the DAC is as good as they claim in regards to jitter rejection that would also be a non factor.
Airport Express to DAC - Cable Suggestion
Can someone suggest a suitable cable to go from the Airport Express out to a DAC. If the DAC has USB, is this the best way or should I consider the optical out to the DAC's Toslink? Bel Canto DAC3. Thanks.