I fly many SLKs and use line from 20# to 1200#.
I have anchored down kites on 20# line and hand flown others on 300# line. It depends on the situation and how I am feeling. But SAFETY must come first.
The key SAFETY question is can you RETREIVE the kite.
The rule would be always set a anchor first before you launch ANY kite(s) that you may not be able to retreive. Then you just need the skills to tie a line with tension on it.
(be glad to help if you need it)
Ability to fly be hand, is not based on line strength but the amount of pull. In 10mph you maybe fine 15 mph and your in trouble
remember you can adjust the bridle point to lower the pull in higher winds
So, to answer your question. Yes you can in some winds you can. But would you want to. You may get tired soon.
I have a 6 stack of gomberg DC at 9' each. Wouldn't ever fly all 6 in a train by hand. But in the right winds, a 3 stack sounds just right.
Oh yes, at the beach you use a sand anchor