Running out of song lyrics that involve three, six, nine and those that are left aren't massively appropriate (looking at you Cat Power... and probably the get low song as well) so instead I'll just point you to this Year 2 Count in 3s homework sheet. It's referred to as a homework sheet, but it could also be an extension or used as an intervention, just any sort of extra practice really for counting in 3s. The activity is made up of two varied fluency questions and a problem-solving/reasoning-style question. An answer sheet has been provided to ensure you can save some time with marking!