The plot twist
Just when children believe one letter equals one sound, along come sh, ch, and th. Name the twist openly: “These two letters hold hands and make one sound.” Children enjoy the rule change when it's explicit.
One at a time
Teach sh first (it's the quietest and most fun - the librarian sound). Then ch like a train, then th with the tongue peeking out. Keep them a week apart so they don't blur together.
Watch for the trap
Th has two versions - thin (unvoiced) and this (voiced). Don't teach that distinction early; just enjoy the tongue-peek. The nuance can wait for the Knowledge Base.
Taggeddigraphsshchth