Quite honestly, the visa officer is unconcerned why you need a 3-weeks holiday. S/he is more interested in why you will return to India. And so far (in this thread), your case doesn't seem convincing enough for a TRV approval.
Your application must address these questions (with evidence):
1. Why not another holiday in Singapore (for example) which is easily accessible, less expensive, no visa hassles, tourist/leisure-friendly? Why Canada?
I can address this in my letter. I chose Canada as I have extended family there who have been inviting me for quite some time and I chose to take this opportunity. I agree Singapore is closer, no visa hassles which is why it's relatively easier to visit which is why I wanted to do a longer holiday for Canada, as it's a harder opportunity to come by.
2. If Canada is your holiday destination then what do you plan to do during your visit? Do you have a well-thought out plan?
Yes, I am detailing points of interest and places I'll be sightseeing in the letter.
3. Do you have the funds to afford your visit + airfare? If mid-way through your Canada holiday you decide to travel to another city, do your finances cover accommodation/hotels without depending on your cousin?
Yes, the financial documents will indicate this. I am including more documents this time, of my FD's, RD's, etc.
4. Will your employer allow you to take 3-weeks off work? Can someone else do your work? Or are you the only one who can do the work you do?
Yes, I have company letter granting me leave for that time which I can attach with the application.
5. Since how long have you been working at your current workplace?
Around 1.5 years now, and have saved up on my leave's for a vacation like this.
6. Do you have a critical work-related commitment which makes it necessary for you to return by a specific date?
My leave is for only 3 weeks after which I have to return to my job.
7. Is the rent agreement only on your name? Or is it shared accommodation? Will you forfeit the deposit if you do not return? Can you prove rent payments for the last 2-3 months?
The rent agreement is only on my name. I can provide rent payment details too, if needed.
8. Do your parents depend on you financially? Do you pay for any of their expenses/insurance?
No, they don't depend on me.
9. Your parents' property + copies of their passports will not help your application.
Ok, I can omit these then.
10. Do you have a valid UK visa? If yes, would you stop in the UK on your return trip after your Canada visit?
No, my UK visa expired. This trip is only to travel to Canada and back.
11. Do you have financial investments? For example, FDs, RDs, shares, mutual funds, PPF, EPF, etc? If yes, are these locked-in or can be liquidated, if required?
Yes, and these can be liquidated if needed.