3 track tunnel portal?

Do you mean a single portal spanning three tracks parallel together as in main lines? Or like this?

Ralph,

Just did this 2 months ago used a Scenic Express PRR style double portal tunnel, and enlarged it by approximately 2.5" by trimming each side by 1.25". Used a tuna can to round out the corners, touched it up with some Tamiya flat grey, and used a #11 exact blade to trim the excess material from the sides. 

Took less than half an hour to make two for each side of a mountain with a three track main line. Enlarged portal measures approximately 11.25", since tracks are on 4 inch centers.

Measure twice and cut once!

Key is taking y

Here are the ones I made by assembling two double portals together. The first is from Scenic Express, the second is a couple of older double portals that I had for a long time. I still need to do a little more to hide the center seams. Good luck with yours.

You can easily carve any shape or size of portal from pink foam insulation. See this thread:

https://ogrforum.com/...44#25504494583650644

I used to eat my lunch right at that spot, and watch for trains, when I worked at Mt. Sinai Hospital, which is a few blocks away.

Here are a couple of ideas from my layout -

Four tracks, and . . .

Three tracks.

Good luck!

Alex