The YN622C is a great product for the price. I do use them for more then a year, and I never had any complaint there about. They are easy to use, and you can use your flash menu of the 5D3 itself as it fully integrates on this point.
Concerning your flash without HSS. Depending on the flash, it might even not go off, as HSS will normally flash a lot of small flashes very short after each other, so that your sensor sees the supplemental light during all positions of the curtain. I'm not convinced that a non HSS flash will go off, when he receives those pulses. If it would go off once, then of course you will see this in your picture. So do use this flash not as mail light but rather as fill light, but even then you could see it depending on the output level of the flash. I would advice not to use a non HSS flash con this combination.