Swings in bankroll

What is normal bankroll variance on a night to night basis? I'm 3 tabling, so I've become accustomed to swings of 3 buy ins or less, but is that normal variance? On average, a typical night will include a swing of plus / minus about a buy in ($10 @ .05/.1) but is that to be expected? I only have about a month of history behind me since the start of multi-tabling, so I don't know if that's normal. Any opinions?