The advantage of double- or triple-board treads is that they drain water more quickly and they’re less likely to cup than single-board, full-depth treads. It’s standard practice to leave a ¼” gap between each board of a double or triple tread for drainage. If using risers, leave a gap of ¼” wherever the tread meets the riser, for ventilation and drainage.
Should you need to make any adjustments to account for variations on the ground, make up the difference on the bottom step. It’s OK if it’s slightly lower than the rest of the steps, provided the others are uniform in terms of rise. Be sure to leave a gap of at least 1/8″ between the ground and the bottom of the first riser.