Just an FYI, it looks like they're releasing the underground/Belvedere tickets for April, May and June on March 14.
"On March 14, it will be possible to pre-book the entrance tickets with scheduled entrance time for the Colosseum, the Undergrounds and Belvedere and the Stern entrance, for the months of April, May and June 2018.
Subsequently, the pre-booking will be available for three months, gradually, to avoid hastened hoarding and grant the availability of tickets closer to the effective date of visit."