15 gallons for going away party please help

ok so I have three slightly dented but still good sanke kegs from a friend who got them from a brewer in Cali. They are all clean and ready to go. I own a cooler, two 5 gallon BIAB bags, 52 quart pot, 25 quart pot, 6.5 gallon carboy and a burner. This will be my first All Grain attempt and I plan to get it right the first time. My question is the recipe I have is for a 5 gallon batch.

7 lbs. Rahr 2-Row Pale
0.75 lbs. Gambrinus Honey Malt
0.25 lbs. Belgian Biscuit malt
1oz Cluster for 60 min
Safale US-05 Ale Yeast. 11g pack
OG 1.040
how do up scale this for a 15 gallon batch? multiply by 3?

My buddy whom is in the US Air Force is going to PCS so I am supplying the beer for his party. Thanks all

Yep, I pretty much always do 15 gallons (or 30 or 45). I take a 5 gallon recipe and just x3 it. The only adjustments I end up making are I sometimes lower the special grains a bit, or the hops a bit. Sometimes if I need 2oz of black patent for 5gallons, I'll just do 4oz for 15, that's how I'll cut back. So those little pops of grains or spices might be potent so no need to straight multiply. For your recipe though, I'd probably just x3 the whole thing. I've also been told (but have not had this issue) that with more grain you can lose a little bit of your OG so it can be better to up the base, that being said, I've never had that happen to me.

You'll also need more yeast (or a good sized starter).

So, if I were you, x3 the whole thing. And dry yeast is cheap, just buy 3 packs.

Roger that, to the brew-mo-bill and off the my LHBS I go