The 7805 is a linear VR. It is not a switching VR.so,the voltage drop accross it becomes heat. If your voltage source is high, yeas you can cascade it so that you distribute the heat among the VRs cascaded. the other parameter is your max current need from the VR. You can parallel and to make sure you will not have one VR feeding the other one when it has a higher output, use blocking diodes in series to each output of the VRs. A better way is to use a bypass transistor with the VR to raise the current. a 3055(NPN) or a 2955(PN) can be used or any equivalent TIP___ transistors. You have to put a bypass Q on all the cascaded VRs. Now the LM350 can carry a higher load. and you can adjust the output by faking the feedback on its sense terminal. So, you can cascade 3 LM350 if you can find them. there is also a TO5 317 which can handle 3A I think. By the way, if you parallel VRs and use blocking diodes, the output regulated V will be less by .7V, so use a 7806 or fake the sensed voltage by putting an R in series on the center leg and an R tothe output to create a voltage divider for the 7805 to put out 5.7V.