---- "If i set the default download resolution to 3K will it affect the quality of the 720p cards?" ----
No it won't. Older cards released in 720p and 1080p will still remain their true quality.
In 3K you can download only those cards, which are released in 3K.
If the card is available in 3K, but your download is set to 720p, there's still noticeable difference in quality of the older and newer (e) cards due to the higher bitrate of 3K.
As Wyld's suggestion goes .. you should download the cards according to your actual screen resolution. If you have a laptop, then 720p should be perfect, otherwise the models will go off the screen. (Don't know about the latest release, though). Desktop PCs with the larger monitors are good to run shows in 1080p or 3K.
Also the the higher quality needs much more space on HDD.