Getter Robo

Getter Robo introduced the combination (Gattai) concept of super robots, with three different pilots and jets able to combine into a bigger and stronger robot and Zanbot 3 is definitely inspired by this. Both plotlines follow the villain of the week structure with an alien race menacing mankind and with a share of dramatic moments on them.  

With teamwork being a theme of Getter Robo , and needing one another for beating the evil villains , and with alt forms both shows are pretty much a must watch 

(in the manga)Getter rays function pretty much exactly the same as spiral energy, the biggest influence for gurren lagann(along with needless and top wo nerae). Also, red mech mains and the drill theme is either from this or needless 

Old mecha with a trio team (the protagonist, the fat guy and the dandy guy) piloting a combining transformation mecha in episodical fights against the bad guys. 

Protagonist has to pilot a giant robot to save earth from episodical attacks by a non-human empire that seeks to conquer humanity. Both are adaptations of manga by Nagai Go that edited the original material into a more kid friendly version. Both are also so old school that they are among the first anime about pilotable mecha. 

Episodical monster of the week format where the protagonist fights against foes from ancient civilization. Practically identical artstyle. 

Both of them are mid 70's anime about a giant robot that transforms from three different parts. Although UFO Senshi Dai Apolon is not a classic anime as Getter Robo is nor has the three different combinations and the interesting characters of Getter Robo, it's a decent watch.