Without seeing the design, its hard to judge. Personally I’ve built sites like that in less then a day, but it really depends what is in the design.
The key is forward planning. Can you show us the HTML version of the site, and we can perhaps offer some guidance on it.
Generally you break the site down into pieces, which become kirby snippets. This is basically anything shared like the header, footer, main menu, social media links. The “shell” you are left with after doing this becomes your template.
You say you have 3 main pages. Are they totally different layouts? It might not be 3 in terms of templates. Basically a template is usually 1 distinct page layout, but technically you can do it with one page template, if you get clever with the logic. It really depends on the site designs. You might have a site with 24 pages in it, 23 are are the same, so you end up with 3 templates, including error page, if that makes sense.
The next step is to make blueprints with custom fields in them to capture the data required to populate the content.
I would start out with Plainkit, add your css to it, and go from there.