In the first two cases it's a different verb altogether that supplies the past tense:
βλέπω (same as in ancient Greek) -- 
είδα from AG 
είδον, from AG root 
Fid- (the same as Latin 
vid-eo)
λέω or λέγω from AG 
λέγω -- 
είπα from AG 
είπον, originally 
έFεπον, proto-Greek root 
eweikʷon
In the third case it is the same verb originally, having taken various forms through time:
έχω from Proto-Greek hékʰō, from Proto-Indo-European 
seǵʰo -- In the past tense formed with the usual 
ektasis (an e- added in the beginning): 
e(h)ekhon > 
eikhon είχον
In the last case it is again another verb that forms the past tense:
έρχομαι (same as ancient Greek) -- 
ήρθα, or 
ήλθα, from AG 
ήλθον, originally from AG root 
ελθ--, which gave also future tense ελεύθ-σομαι > ελεύσομαι (it's the same root that gave ελευθ-ερία, the act of coming and going freely).