Yes every designation is a separate entry unless your company changed the name of your designation just like that. Example, my bank changed the name of our position name from Personal banking officer to Financial Advisor for branding purposes.. same job, same duties, same life, same salary.. nothing changed except for the titles outside our offices and our CEO lol
To answer your 1st Qt about the letter: When I said 3 pages, its not because of 3 positions...it was because my employer kind of acted like a govt office where they have specific formats coming from the top with your supervisor having no flexibility to create an employment letter of his own imagination. Thus it was detailed with awful lot of crap not really needed. Its totally upto you whether you want 2 pages, 3 pages, 4 pages or 10 pages for your 3 positions. It should follow the guidelines mentioned in this link under section proof of experience.
http://www.cic.gc.ca/english/resources/tools/perm/express/intake-complete.asp
To give you an example:
JSPCAN worked in COMPANY A and held positions A1 (2012) AND A2 (2013)
JSPCAN worked in COMPANY B and held positions B1 (2014), B2 (2015) AND B3 (2016 to current)
For express entry, JSPCAN would approach COMPANY A's HR/supervisor for ONE letter providing job details, resp, salary etc for each of the positions A1 and A2 with the relevant dates. JESPCAN would upload that SAME letter in each of the two entries A1 and A2 as express entry will generate uploading of docs for every entry.
JESPCAN would also approach COMPANY B for ONE letter giving out details for all 3 positions held there with relevant dates start to end or if still working then as start date to current etc. Again that same letter will go in 3 entries for those 3 position.
TOTAL letters :2 TOTAL WORK EXPERIENCE ENTTRIES IN EXPRESS ENTRY PROFILE: 5
If you find it odd to upload the same letter in diff spots, you may do what I did. I created different pdfs for every entry containing the same letter but with different payslips. Example to show COMPANY A work experience, JSPCAN would create 2 PDF files,
PDF #1: letter + payslips for A1.
PDF #2: letter + payslips for A2 and then uploade those pdfs in relevant position spots i.e A1 AND A2.
sorry if its too detailed and confusing. hope this helps