Excessive moisture is sure death for Helleborus.
I grow Helleborus under Oaks, Sweet Gum, Pines, Maples, Magnolia (deciduous), Sourwood, Cherry and Japanese Maples, but all are in raised beds, the red clay soil amended with generous quantities of compost and composted ground bark to improve drainage.
I have no full sun location in the garden, except during the winter, but some receive several hours of morning or noonday sun or bright indirect light during the summer. That will encourage better growth and more flowers.
They will grow under low, evergreen trees or shrubs, but blooms will be sparse, if any at all.
Before planting the Helleborus, would advise conducting a percolation test for soil drainage. Prepare the planting hole and then fill with water. Check the time required for the water to completely drain. If more that a few hours, you will need to modify it by digging deeper and adding crushed stone or coarse gravel at the bottom.

Purchase only healthy, potted plants. Avoid dry bare roots or one's without foliage. Success rate with those will be rare or nil. When planting, do not disturb the roots, they resent that and the plant will sulk for day or weeks.
Keep the soil moist (not soggy) until they become established for one growing season. After that, they are fairly drought tolerant and will only need irrigation during prolonged periods of no rainfall.
If you amend the planting area with compost, it isn't necessary to fertilize them. My red clay is very acidic, and when planting, I sometimes add about half-cupful(4oz.) of Dolomitic (powdered) Lime, mixed into the planting soil.

Keep the roots moist, but never soggy, give them plenty of bright light and you will be rewarded with colorful winter blooms!
Rb

Thanks razorback for the information. I think I'll add some more topsoil to the bed under sweet gum. I just got a bag of top soil and peat something. I have a hosta that's okay there but sure a little soil wouldn't hurt it. Not worried about water moisture or drainage, the sweet gum takes care of that. The helleborus are from a trade I just received. Now if I could get a few days without rain.