They do not need to be exactly equal volumes, but I've never had a recipe where there was that big of a difference between the mash and sparge volumes. Here's how you'd normally calculate how much mash and sparge water you need.
- Typical mash can range between 1.25 qts water per lb of grain to 2 qts/lb. At 3 gallons for 9 lbs of grain, BeerSmith is giving you 1.33 qts / lb - that's within a typical water: grain ratio.
- Typical absorption = 0.125 gallons per lb of grain. At 9 lbs of grain, you'll lose approximately 1.125 gallons of mash volume to grain absorption.
- That means you'll be yielding about 1.875 gallons of "runnings" from your mash. If you sparge with 4.5 gallons, you will have 6.375 gallons of wort heading into the boil kettle.
I typically try to get about 6.75 to 7 gallons pre-boil for a 5 gallon batch. I also typically have a heavier grain bill with a higher water : grain ratio (1.5ish usually), so I have never had so little runnings coming out of the mash.
So, if it were me, I'd adjust my mash volume up closer to 1.5 to 2 qts / lb to get more runnings from the mash (so maybe like 4 gallons into mash?), and reduce the sparge accordingly to end up with around 7 gallons heading into the boil kettle.