it (used to refer to a thing previously mentioned)

यह (जिसका पहले उल्लेख किया गया है)

English Usage: "It is a beautiful day."

Hindi Usage: "यह एक सुंदर दिन है।"

is (3rd person singular of be)

है

English Usage: "She is a doctor."

Hindi Usage: "वह एक डॉक्टर है।"

as (used for comparison or similarity)

जैसे

English Usage: "He runs as fast as he can."

Hindi Usage: "वह जितना तेज़ दौड़ सकता है, दौड़ता है।"