Please bear in mind that all the three trains you mention run on routes that are traversed by plenty of ordinary regular trains without the need for reservations or extra fare supplements. This can help if you are travelling on a ‘budget’.
Try not to focus too hard on the so called ‘scenic’ aka ‘panoramic’ trains. Nearly all railways in Switzerland are scenic in some way – if you don’t see high mountains you will see other icons of Switzerland such as lakes, rolling pastures, farmsteads, cows etc. – so you will rarely be disappointed with any Swiss train trip
If you get a Swiss Travel Pass, you can travel on any and all of the railway lines of the main Swiss national network, this includes any train on the so called ‘Glacier Express route‘ (which in fact is just a normal railway with lots of normal trains services throughout the day.
In particular the Gotthard Panorama Express (see below) is not worth the bother in my opinion.
Parts of the Glacier Express are also mundane, and if you choose your itinerary carefully you can ride the spectacular bits of the so called Glacier Express route on ordinary trains as day trips without luggage and be able to get off and enjoy the mountain air, rather than just see the views from inside a sealed train.
Gotthard Panorama Express – a personal view
This is ‘take it or leave it’ advice:
Personally I am sceptical of the attraction of the Gotthard Panorama Express. It is an attempt to keep some tourism going on the Gotthard ‘mountain route’ since the rest of the fast trains now go through the base tunnel. However, you see mainly valleys not high mountains. Also it is reservation obligatory and first class only.
Also, the train gets rather lonely – part of the ‘romance’ of the Gotthard route previously was that your train competed for space on the line with huge long international long-distance trains carrying 000s of passengers a day between northern and southern Europe, and those passenger trains interacted and weaved in between dozens of heavy freight trains lumbering back and forth across the Alps.
The curiosity was seeing another train high above you one minute then a few minutes later passing by your train on the same level, then perhaps seeing it again below you a few minutes after that. It is this that no longer happens as there are so few trains left on the line.