Iran is denying and will deny involvement. They have been in constant contact and active with both Russia and Hamas during the two years of planning the weekend's atrocity.
Russia will have had more sway with Iran before relying upon them for weapons to support their invasion of Ukraine but now Iran will have a stronger voice. This Russian reliance is in part because Russia has been isolated from the West due to its war on Ukraine. They have a strategic alliance with Iran and so planning on such matters, especially weakening the West, tactics to weaken the West's support for Ukraine will be tactical considerations.
The US has already started delivering weapons to Israel. Will that impact their ability/desire to supply both Ukraine and Israel, especially if Israel is pulled into a wider conflict, probably. Would Putin see this as a positive if somewhat risky move, well that is up for each to decide for themselves but I believe so. Will Hamas and Iran discussed in meetings with Russia over this last year- I believe so. An actor doing something crazy and horrific could see them isolated (i.e. Russia) so sign-off will have been required. Hamas is saying they didn't tell their allies in other Palestinian factions, as well as in Hezbollah, Iran, and Russia until 30 minutes after the attack began. I'd say that is scripted and they all knew, after all, Iran fund Hamas to the tune of $100 million per year if reports are correct.
There is a much bigger geopolitical game of chess in play and it will be no surprise to find that the Hamas foot soldiers and the poor people of Gaza have been used as expendable pawns.