ARD 3.8 is current, obviously.
I'd doubt that ARD 3.0 works here, as 3.0 is comparatively ancient, but — since you're probably not inclined purchase an upgrade to current from the Mac App Store — generate a scratch OS X 10.11 install on a scratch disk and install ARD 3.0 there, and see if it works. Or install an older OS X release on the external scratch disk — whatever old release your Mac will boot and that supports ARD 3.0 and at least OS X 10.6.8 with Mac App Store — and see if you can get MAS to detect and recognize that and put the product in your purchase history for a subsequent upgrade on the not-scratch boot disk. Or call Apple Support and explain your situation and see if a redemption code can be made available. That lattermost approach might be quickest.