So I guess su-8 for future balanced upgradability?
Personally I'd pass. The OLED screen is small and like all OLED, prone to burn-in, and the on-device controls are nearly non-existent. Also the ESS IMD Hump is still a factor, and also the device is due for a refresh if you ask me (devices with bigger displays and newer chips should be on the way, with hopefully addressed "ESS IMD hump" issue).
Also subjectively speaking, the device simply looks like a mix of boring/outdated to me from an aesthetic point of view.
The Topping D50 similarly as well.. The screen is better, but is now an "old" device with respect to aesthetic language. Also no balanced, so a massive deduction there compared to the SMSL. If going for the D50, might as well save $50 and get a device like the DX3 Pro with more features (and one more Coax) and since it doesn't use an ESS chip, no ESS IMD hump issue to worry about, etc..
But to be PERFECTLY honest, I'd wait completely and not get either 3. The DX3 is looking like it might get a refresh due to some instances of failures occurring for a few people. The DX50 doesn't make much sense to me, and I'd not get it due to the existence of the SMSL SU 8 that has balanced and costs the same. The SMSL SU is similarly bland-ish looking, and that screen is nearly pointless at that size.
Just hold out, or tell me what it is you value most and I could make the choice easier for you. Otherwise, I don't have any actual metrics to go off of. Generally speaking though, all three are great products in their own right. But when competing with each other, and at this moment in time relatively speaking, each has severe downfalls for me personally if you're looking to make a purchase today. (Though my brother has the DX3 Pro, and that one I feel makes the other two irrelevant is price is a factor, and balanced isn't mandatory).