- I congratulate you from the depth of my heart.
1a. I congratulate you from the depths of my heart.
- Thank you from the depth of my heart for your kind words.
2a. Thank you from the depths of my heart for your kind words.
- I love him from the depth of my heart and soul.
3a. I love him from the depths of my heart and soul.
Are all these sentences correct grammatically?
‘Depth’ is usually used in the literal sense as the noun from the adjective ‘deep’ as in: I have measured the length and the depth of the hole. ‘Depths’ is more often used in the figurative sense as you have shown in 1a/2a/3a. Having said that, I would add that it is more common to say: From the bottom of my heart.
Only ‘the depths of my heart’ is correct in all those.
When relating to the deepest part of something the noun is usually presented in its plural form. There are exceptions to this (depth of feeling), but 'depths of (the/my/your, etc) heart is not one of them.