Here is what Bruce B said in another thread but I'm not sure if it relates to your situation with three stereo DACs:
Well I'll tell you the truth... I have had every clock in here to test from the Big Ben to the Antelope Rubidium clock and neither has made a difference. You always... always want to use the internal clock of a DAC and make it the Master... IMO! I've tried using the Lynx Synchro-lock on their card and other such ways, but when you only have 1 converter in your system, always use the internal DAC clock. Manufacturers that use and sell you an outboard clock are just making money off of you... is the clock in their own DAC broken? Did they skimp on the clock inside?
Unless you are recording with multiple converters, such as in a studio or location setting, I never use an outboard clock, period!