90% of my vaping is relatively high powered DL vaping and I always buy either 50ml or 100ml shortfills - and I add one nic shot to a 50ml shortfill and two nic shots to a 100ml shortfill to get either 60mls or 120mls of 3mg nicotine juice.
I'll happily use either freebase or salt nic shots, or even one of each in a 100ml shortfill if that is all that I have available. When you are mixing juices to 3mg nicotine personally I wouldn't obsess about which type of nic you are using because I find it makes FUCK ALL DIFFERENCE.
If you are buying juices with a higher concentration of nicotine for MTL vaping then you will DEFINITELY notice a difference because Freebase nicotine will give a throat hit, Nic Salt doesn't - you will definitely find you prefer one type of nicotine over another. If you like a throat hit (similar to a cigarette) the chances are you will prefer Freebase, if you don't need a throat hit you'll probably prefer nic salt.
For DL vaping you probably don't need more than 3mg, if the type of nic has any throat hit or not doesn't particularly matter. Most DL vaping is at much higher power settings, you are inhaling much more vapour, and you're not strictly mimicking the actions of a cigarette, so I recommend you don't give a shit about what type of nic shots you use, Freebase/Salt Nic work just as well when making LOW concentration nicotine mixes like 3mg.
I think what is fucking with your head is you are vaping 'ice' type juices, or something some twat has added an 'ice' nic shot to and you're having a tough time vaping it. I don't blame you - Ice and menthol juices should be used for drain cleaning.
For what it's worth I only vape nic salt MTL juices or shortfills with Freebase nic shots (because they are easier to acquire and tend to be a bit cheaper) - but if I have any nic salt shots available they work just as well. Just don't buy ice shots lol.