Morning Jim. Three sets of 15 rides is a lot to set up but it should be possible. Most I've ever setup in a league is probably 6 rides but it should be just a case of repeating the process for each ride. I don't know the specifics of your rides so can't advise on certain details such as whether you want to manually approve each attempt or not.
Anyway, here goes:
1/ Go to the main page of your new group.
2/ select leagues from the menu on the left of the page.
3/ Select New League from the menu across the top.
4/ Give it a name and if you want you can add a description.
5/ Hit the Create button.
6/ You now have to add the stages so select Add New Stage.
7/ now give the stage a name. I usually put Stage1 at this point. Description if you want. I don't usually bother.
8/ Select the date you want the league to start and that must be after todays date. If you want the first stage to be live for the whole duration of the league then select the end date to be the last day of the league. This is what I always do with the CG. It's worth working out all the dates before setting up the league as it'll be much easier to input.
9/ Validation rules: to try and avoid having to validate each ride do the following:
Change average heart rate to 0
Change Lost Signal to 9999
Tick the Coherent weight box
Then hit save.
10/ Now you need to add the ride for the first stage so select Create New Session.
11/ Now the hard part. You need to find the ride but sometimes in my experience the search does not find the ride so easily so you need to persevere with different filter settings. Trial and error is best here. If you want a 3D ride make sure you have the 3D box ticked. Use Difficulty to find steep climbs and hopefully the keyword search will help.
12/ When you've found the ride select Setup then Schedule and the ride will be added to the stage. I've noticed they've just made a change here and it now looks like you need to select Setup first then Details and you have to change the type of session to League Master Session and then select the name of the league it's to go in. Then Schedule.
13/ Now the first stage setup is complete you need to add the second stage by selecting the greyed out Unpublished across the top. Then select the league name on the left. Select Add New Stage. Then you'e back to the bit where you add a name and dates for the second stage. The start date might be a week after the start of the first one as per CG. Go through all the above again for all the stages.
After all the stages are attached to the league select Unpublished again. Then select the league name. Then select Publish. Because you can only have one league per group running at one time you need to wait for any previous league for that group to end before you select publish.
It sounds more complicated than it is and only practice will get you there as it eventually did for me.
Good Luck Jim. Let me know if you have any difficulties and I'll try to help.