It's common for programs to have less deadlifting when compared to other exercises, just because it puts a lot of stress on your lower back.
Since you rely on your lower back in every day life, you run a greater risk of over-training it, and developing persistent back pain. Especially on a program like Stonglifts that already has you squatting every workout.
Also, rep ranges are generally chosen in order to control the amount of weight you lift in a set. You can lift a weight 3x in a row, but that same weight might be too heavy to lift 5x in a row.
Right now, your probably using sub maximal weights so it's not a big deal, but it's something to keep in the back of your mind a few months down the road when you start hitting plateaus. It will probably be best then to switch back to 1x5.
Thanks for the response. I am definitely sub maximal weight haha. I also feel like I can keep my form better when I do 3 deadlift reps instead of 5. I think I'll stick with 3x3 for awhile, but I will keep your advice in mind.
I have another, possibly stupid, question. Are all barbells 45 lbs? I was deadlifting 155 lbs today, and it felt surprisingly light. My first thought was "getting strong af", but when I finished I noticed the bar seemed skinnier than most. Maybe it wasn't a 45 lbs barbell?