You will need 4 bottles, you can generally get a little more than 3.1l in it if doing it on axle stands as the fill plug is on the front of the box it won't be level with the front picked up.
Postage doesn't cost anymore for 4 bottles rather than 3 I don't think? And halfrauds want far more for the oil than opie do.
You'll get away with three 1 litre bottles mate
I also managed to top mine with a flexible hose and an funnel with the car back on level ground after draining the old stuff and replacing the gearbox plug washer.
This was to get an accurate level, all i needed to do is remove the airbox to gain access.
You can also remove airbox form time to time and remove the fill plug and stick a finger in to see it the gearbox oil is still at the maximum (inline with bottom of fill plug hole), just put your finger in the hole straight and bend it down slightly if it comes out with oil on it your still up to level
I used the Castrol EPX75w-80 stuff from Opie Oil about 5 months ago and all has been fine so far.
For my next Gearbox oil change i will probably use Castrol again, i would used the Fuchs as mentioned above but i know how temperemental the 172 gearboxes are