These would have been higher than average page counts, but certainly would have worked out better that 3 thin volumes.

Captain America volume 7
Collects Captain America #149 - #162
May 1972 to June 1973 - 296 pages of comics

Captain America volume 8
Collects Captain America #163 - #176
July 1973 to Aug. 1974 - 277 pages of comics

Captain America volume 9
Collects Captain America #177 - #192
Sept. 1974 to Dec. 1975 - 304 pages of comics