That is a beautiful elderberry! 

I don't think you will kill it if you do a hard cutback. If you are feeling unsure, you can cut the tallest trunk to the ground and leave the remaining two. Most likely, new growth will come up from the roots. If you cut the trunk to 1-6 inches from the ground, it might pop latent buds on the trunk. These might have a weak connection to the trunk and be prone to breaking as they grow taller and heavier. Growth from the roots should be strong.  

Next year, you can take down another one of the large trunks. And then on year 3, the final large trunk. That should give the shrub plenty of time to push new growth. 

You can then have a 3 year pruning cycle where you remove a third of the growth on an annual basis. This should keep the shrub shorter and at optimum fruit production. It will also encourage good airflow to reduce disease pressure. 

I recommend putting a cage around the tree to prevent deer from eating the new growth. Galvanized livestock fencing, purchased in a roll at farm or feed stores, is an option. 

Thank you for all this information. I was so worried about using a chainsaw on the elderberry.

If I understand correctly, any hard cutback and hard pruning I do, should be done in late winter or early spring, right?

Late winter/early spring (March-to the first weeks of April, weather patterns dependent) is ideal, but not the end-all, be-all. If summer is the only time you have to cut it down, then that's probably okay. You will most likely get new growth starting immediately. The concern would be that the new growth it doesn't have time to harden off before winter and it might suffer die-back. 

Personally, if I had to cut it back this year, I would wait until after the fruit has ripened so that the birds can benefit from the food. 

If you can wait until March/April 2026, that's the best plan.