never do solo mining before, if I stop the card for a while(eg. electricity cut off). When I restart, am I losing all the previous work?
Its statistics. If you e.g. find 5 blocks in 1 day, than you probably wouldn't find another whole month

But on the longer run, you cannot cheat it, it is the time that you've been mining that it counts
still a little confused.
1. at present,because of the hot days, I usually shut down my machine when I was at home and turn them on when I'm off, let's say 12hours/12hours, is this mean on the long run i'll get half compare to let the machine run 24 hours/day without stop
2. I got 3 machine, which will be better,pointing the 3 machine to one wallet adress or 3 different adresses, or just the same