If you have tried the other 3 starters and getting the same sympton i would discount it and start as said above looking at the power feed to the starter.If the power feed is defiantly OK and the starter's want engage is there something wrong with flyweel. Take starter off and try bumping it.It should bump easily.If it sstarts then it starter.If you try bumping it with starter in place it may jar starter and then work fine.Should bump easily enough with starter in place so would be my first port of call.HTH