I'm going to make some assumptions.
If officer no. 1 had thought for a minute that the second girl was armed, I assume he would not have been rolling around on the floor with the first girl, leaving them both at risk of injury from girl no.2 with her mythical knife.
I assume that in the minutes prior to him commencing rolling around on the floor with girl no. 1, the first officer called in the incident and requested assistance. I'm assuming at that point, he would have notified his colleagues whether he had any suspicion of either of them being armed and warned them of such risk.
I assume that when officer no. 2 (female cop) arrived at the scene, if she had any information whatsoever (either radioed through or shouted out in the moment by officer no. 1) that girl no.2 was armed, she would not have joined him on the ground, but instead headed straight to girl no. 2 to restrain her.
I assume officer no. 3 arrived with officer no.2 and had the same information she had.
I assume they all have extensive training on proper restraint methods and on the spot risk assessment. I assume they are all aware that the very special powers that are conferred to them and paid for out of the public purse require them to not react as mere members of the public would possibly react (scared, knee jerk, thoughtless, reckless, gratuitously violent, angry etc); but to actually conduct themselves in the manner in which they have been trained - which, yes, should be better than the rest of us, because that's their job and that's why they have powers the rest of us do not.