I cant find any reservations for October 3& 4th. Is it because its filled or too early to book for any of the above? Thanks!
For Gaudi Park you should be able to buy tickets in about two/three weeks on its website.
For Sagrada Familia they should be available on the official website about two months before your desired visit.
For Monserrat I don’t know because for our group of 7 a used a tour operator which included round trip transportation from my hotel.
Make sure you are purchasing tickets at the official websites for these venues. Here is the website for Park Guell looks like they only sell tickets up to 90 days in advance. Here is the website for the Sagrada Familia looks like they have tickets up to 68 days in advance listed (Aug 31st as of today).
Monserrat may have a similar limitation on tickets. We didn't visit there so no actual experience with the venue/website.
Thank you for everyones reply. Very helpful.