The "three meals a day, no snacking" does NOT apply to pregnant women or breastfeeding Moms. We need to get our calories in where and when we can, so grazing, snacking, or eating mini-meals is more than acceptible if that's what it takes to get food into your sleep-deprived, feeding-every-three-hours body.
Sipping on a can of coconut milk or snacking on olives throughout the day is a good way to get extra calories in, too. I eat a can of olives pretty much every day. Nuts and seeds are a good calorie-dense source too. I know we say not to crack out on them, but when it comes down to needing to get extra calories in, it's A-OK to rely on these cal-dense sources. Of course, if you find yourself blowing through a jar of Sunbutter every three days, maybe it's time to pull back on that one.