It's hard to say how much traffic you'll lose from the merge. Like Logan said, you'll definitely lose a bit when you first move, but long term, you'll need to look at your competition to figure out if it's better to keep the pages separate or combine them.
I don't recommend keeping pages A, B, and C if you're going to hide them from the main structure of your site. Pages get most of their Page Authority from internal links (unless they're link bait), so they won't be able to rank anyway.
That said, here's how I'd estimate the loss of traffic from the move:
- Use Google Search Console to determine the primary keyword/s for page A, B, and C
- Use a tool like Open Site Explorer to determine the number of links A, B, and C have. (Bonus: look at the websites linking to A, B, or C. If those are resource pages, there's a good chance their webmaster will update their links to page D, which will help with the traffic dip. If they're from news articles, you'll probably have to rely on 301s.)
- Search for each of those top keywords and look at your competition. Does the competition closely target the term? Will page D seem as relevant to the keyword as A, B, or C did?
- Now, look at the Page Authorities of the competition for each keyword. Will page D, which will have a combo of links from A, B, and C, blow your competition out of the water? About match it? Still be a bit behind?
- Here's the part that's really tough: for each keyword, estimate where page D would rank, given how well it targets the keyword and how many inbound links it has.
- Estimate the % increase or drop in traffic based on adjusted click through rate. You can find this by playing around in Google Search Console to find a time when your site ranked in a different position, or by using average click through rates, like here.
- Once you're done, put together your estimated percent increases or drops in traffic to estimate how the new page will perform. (I recommend you look at a percent change because adding up totals only for top keywords won't take long tail keywords into account, and you'll almost definitely come up with a much lower count than you're currently getting.)
Not the easiest process in the world, and your estimate will almost definitely be wrong, since you make a lot of assumptions along the way. But it should give you an idea of whether you'll eventually gain or lose traffic from the move, once that initial Googlebot confusion wears off.
Hope this makes sense! Let me know if you have any questions!
Kristina