To determine the correct statement about subcutaneous nodules in Rheumatic fever, let's examine each option provided:

1. They are present over flexor aspect of forearm:
   • This statement is incorrect. Subcutaneous nodules in Rheumatic fever are usually located over extensor surfaces such as the elbows, knees, and sometimes over the spine and scalp.
2. They are painful tender nodules:
   • This statement is also incorrect. Subcutaneous nodules in Rheumatic fever are characteristically painless and non-tender.
3. They typically appear more than 3 weeks after onset of other clinical manifestations:
   • This statement is the correct answer. Subcutaneous nodules most often appear a few weeks after the initial symptoms of Rheumatic fever, such as fever or arthritis.
4. The usual size of these nodules is 3-5 cm:
   • This statement is incorrect. The nodules are generally smaller, usually ranging from 0.5 to 2 cm in diameter.

Thus, the most accurate statement is that the subcutaneous nodules in Rheumatic fever typically appear more than 3 weeks after the onset of other clinical manifestations.