Here is with some selected large and long extensions (*) added:
(Please point out any I missed.)
Going by just free agent contracts of 3 or more years and adjusting for inflation, here are the years we made the big splash signings:
1991 Matt Young 3
1992 Frank Viola 3
1999 Jose Offerman 4
2001 Manny Ramirez 8
2002 Johnny Damon 4
2004 Keith Foulke 4
2005 Edgar Renteria 4, Jason Varitek 4, Matt Clement 3
2007 JD Drew 5, Julio Lugo 4, *Ortiz 4, *Beckett 3
2008 Mike Lowell 3
2009 *Youkilis 4, *Pedey 6, *Lester5
2010 John Lackey 5
2011 Carl Crawford 7
2012 *Adrian Gonzalez 7, *Buchholz 4
2013 Shane Victorino 3
2014 *Pedroia 8, Castillo 7
2015 Pablo Sandoval 5, Hanley Ramirez 4, *Porcello 4
2016 David Price 7
2018 JD Martinez 5
2019 Nathan Eovaldi 4, *Vaz 4
2020 *Sale 5, *Bogey 6
2003, 2006 & 2017 are the only 3 seasons since 1999 where no player was signed or extended to a 3+ year deal of significant value.
Years with 4+ year deals:
99, 01, 02, 04,05, 07, 09, 10, 11, 12, 14, 15, 16, 18, 19, 20
Years with 5+ year deals:
01, 07, 09, 10, 11, 12, 14, 15, 16, 18, 20
Years with 6+:
01, 09, 11, 12, 14, 16, 20
Years with 7+:
01, 11, 12, 14, 16
Years with 8 years:
2001 (Manny)