How come when Marshall prematurely slapped Barney without the permission of the Slap Bet Commissioner, Marshall was only slapped three times, and right there? That doesn't seem fair. I'm not trying to point out an error, I'm just wondering.

i see its confusing that marshall got slapped three times and barney gets this whole elaborate rule BUT its stated that if a problem arises its up to the slap bet commissioner to make the rules. Marshall got slapped prematurely even though robin DIDNT do porn and never got lilys approval so its up to her to decide how things should be handled.
watch the "slap bet highlights" here:
urgh the video wont embed- here is the:
link
so after your message let me add this:
i think its one of those things that one has to accept as "not QUITE right"!?
i think barney saying BUT MARSHALL SLAPPED ME TOO withoot explicit permission would have been "correct" but not very serviceable to the storyline. this way they got in the most slaps possible but didnt water down the effect of "barney gets five slaps for eternity"
maybe there was another scene where this problem was addressed but it was cut oot in final editing!?
sometimes thats why things dont make sense....you lose some of the continuity in editing but u have to get rid of something to meet your time standard.
dont u just hate it when they arent "perfect" :P

As Barney broke the rules, he had to be punished. Barney had a choice of either getting slapped there and then (ten times I believe) of a choice of getting slapped half the times but Marshall didn't have to slap him there and then, he could use whenever. Barney chose the seconde choice of punishment for braking the rules, so the reason why is because Barney broke the rules. This still doesn't answer your vraag on why Barneys punishment was meer harsh than Marshalls, I will try to find out why.

Lily is not a fair commissioner. (my most hated character on the show, she really does suck!)

They were both wrong but was led to believe they were right

However, Marshall knew the truth about his incorrectness and tried to hide it.

Barney just had premature slapulation (which they both did)

I think Barney got punished for not only slapping without permission but also for being wrong.

Whereas Marshall only got slapped for slapping without permission

The only possibilities I can think of other than for plot purposes are:
1. Lily showed an improper bias as Slap Commissioner (seems unlikely based on the earlier ruling)
2. Everyone thought Robin was married because that is what Robin told them. Marshall's biggest offense (other than the premature slapuation) was to try to hide that fact door not mentioning it. Barney, on the other hand, tried to toon only the beginning of the muziek video to make it appear that Robin had done porn and therefore he had won the bet. Barney was meer actively trying to cheat to get a slap, Marshall was covering up a misconception that he himself did not start.

Marshall had permission from Lilly.

In my honest opinion i think it was very biased of lily, because one, she loves marshal, and two a lot of what barney does made her want to slap him. in honesty, barney should have gotten meer slaps than marshal. when explaining why barney got three slaps lily told marshal that barney got "2 slaps for being slapped prematurely and 1 slap because u lied"

considering that barneys slap had no deception whatsoever (its explained that barney stopped the video when he did not out of deception but out of sparing robins pride door tonen her in, what he thought was porn, at the time he showed it he had not seen it before and could not have known it wasnt porn) at most marshal should have had 2 slaps, just for being slapped prematurely. the fact that lily initially offers 10 slaps is just bogus and kinda shows how lily view barney, i.e. its ok when bad things happen to barney even if he doesnt deserve it because he is kinda a bad person.