I've never seen one. This may be because the prototype (in the UK at least) never went in for three-track tunnels, as far as I am aware, other than as 'cut and cover' type tunnels on the approaches to stations. But I wouldn't be surprised if someone on the Forum comes up with an example!
At one point I was looking to make a 3 line tunnel but plans changed long ago. However I did get a metcalfe stone double tunnel portal with a view to kit bashing it. I had seen it done elsewhere. May be something to consider beyond scratchbuilding one
ps, i have a unused metcalfe stone double tunnel portal for sale if you want it ;-)
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