Well the problem against Tsunade that Orochimaru has is that many of his seen attacks deal with being poisoned or stabbed....with Tsunade's Byakugou she is capable of negsting all of his techniques...she also had that nerve switch move she used on Kabuto....now Orochimaru could easily figure it out, but in that time it could be over.
Now of course Orochimaru has to be caught for Tsunade to even hit him, but all she simply has to do is wait for him to attack....if he tries to stab her with his sword then she csn easily grab a hold of him right then and there....Orochimaru afterall mentioned she csn kill in one hit....and it has been seen exactly how easily she can do it too (messed up buildings and shit).
As a whole Orochimaru is most likely the strongest and smartest of the three, but it all comes down to match ups....
Orochimaru < Tsunade < Jiraiya < Orochimaru.....the signs just indicate bad match up....
For example, Jiraiya has several long range techniques he showed as well as physically enhanced Sage mode do he is capable of defending Tsunade's heavy hits while keeping a distance to wear her down.
Jiraiya's techniques are incapable of actually getting Orochimaru due to his shedding techniques
Tsunade has the advantage due to many of Orochimaru's techs being piercing and stabbing related (snake related) She is strong enough to handle summons, and dodge snake armies or just run through them, and if Orochimaru tries to stab her up close she survives due to Healing and she has him in her hold.
No I mean Edo Tensei kinda changes things up a bit...but as far as just Orochimaru goes His attacks (so far) don't exactly kill Tsunade.