@viciousviscosity Looks like "could" comes from "cuðe" which was the past tense of the ancestor of "can". In the 14c., The ending changed to just -d and in the 15c. the -l- was added but it's ahistorical probably by influence of the other two.

"Should" comes from "sceolde" which was the past tense of the ancestor of "shall" and "would" from "wolde" which was the past tense of the ancestor of "will". In both cases the -l- was originally pronounced.