I was trying to build a budget trainer plane the most expensive part is lipo battery I can only invest 800 rs in a single battery but it has only 850mah capacity 3s 

Basically I am building a trainer plane which gonna fly slow so can I use 2500mah ×3  18650 s
In an pack ?

Will it fly Help

You can definitely use 18650 cells for an airplane, heck I have used them in a quadcopter for photography and it worked perfectly.

However, you need to use the correct cells, 18650 is a benchmark for size, however, there are many manufacturers making 18650 cells that vary slightly in battery chemistry, capacity and discharge rating, hence you have to be careful while selecting the cells for the battery. I can share some insight on the battery that I've been using for about a year now.

Battery: Panasonic NCR18650B
Grams per Watt-Hours is 57% that of a typical LiPo
Pack Capacity: 3.3Ah (3,325mah)
Pack Voltage in 4S: 14.4v
Pack Power: 43 watt-hours (personally tested with a wattmeter)
Pack C-Rating: 2 (6amp max discharge rate)
Pack Cycles: Hundreds! I've not personally tested this, but this is the same chemistry as a Lenovo ThinkPad, which can last years of daily use.

this are the cells am talking about 3400mah

https://www.fabtolab.com/panasonic-18650-battery

am unable to find lower prices anywhere else in India, if someone manages to get these cells for lower prices, please update me