music creator 6

Yes because Cakewalk Sound Center is not multi-timbral. FWIW, because it is not multi-timbral there is no need to worry about MIDI channel assignment at all. Each instance of a synth can work on any MIDI channel.  The only synth bundled with MC6 which can play more than one instrument at a time is TTS-1.

Yes, three instances of CSS would be necessary to play three different instruments at the same time. As I mentioned earlier the only synth bundled with MC6 which can play more than one instrument at a time is TTS-1.  Is there a problem using multiple instances of a synth?
 

You can always freeze or bounce and archive tracks to free up resources. The best source for software suggestions is the software forum. I don't do orchestral work but I believe you are going to find the better ones are expensive and very resource demanding. Buying the industry standard sampler Kontakt may be a good start. Like CCS, a sampler is limited by the quality of the sample library. Unlike CCS, Kontakt is multi-timbral. You may also find yourself hitting the limits of MC6 pretty fast.

Welcome to the forum.
The 'limit' scook refers to is only 8 synths per song. Easy to get around by bouncing the synths and/or using multi-timbral soft synths.

If you take the time to learn how to assign the channels in TTS to midi tracks, you can have one instance of TTS and have it playing 16 different instruments.... going to the full limit of 8 instances of TTS..... and the max midi track count permitted in MC6 of 258..... the math works perfectly. You can have 8 TTS instances playing 258 different midi tracks on unique channels..... assuming your computer can handle that load.
 
You can also have a combination of CSC and TTS to allow the background stuff to use TTS and the solo instruments play in CSC. Also.... bouncing the midi to audio and then using that synth for another midi track is a good idea. The computer can generally handle more audio tracks than midi since very little computer power is needed to play audio whereas lots of things have to happen in midi to get the sound to the speaker in sync....
 
On my web site (link below) in the recording section, there's a section on Advanced TTS where I show you how to set up 3 channels in TTS to play 3 different things.... just follow the directions and ask back here if you have questions.... repeat the process all the way to channel 16 if that works for your needs.  To me, the biggest downside to TTS is that it uses GM voices and while they sound good, sampled synths can get a much better quality sound.
 
BTW: In the years I have been here, I have heard several very impressive classical orchestral compositions done using only TTS and the general midi voices it uses.