Factor + combinators.extras, 42 bytes

0 [ read1 ch>lower 96 - dup . + ] thrice .

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• 0 Push 0, our sum, to the data stack.
• [ ... ] thrice Call a quotation three times.
• read1 Read one byte from standard input.
• ch>lower Make it lowercase, e.g. 65 -> 97 or 97 -> 97.
• 96 - Subtract 96 from it.
• dup . Output on its own line non-destructively.
• + Add it to the sum.
• . Print the sum.

Factor + combinators.extras, ⁴² 35 bytes

0 [ read1 32 mod dup . + ] thrice .

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-7 bytes thanks to @MarcMush

• 0 Push 0, our sum, to the data stack.
• [ ... ] thrice Call a quotation three times.
• read1 Read one code point from standard input.
• 32 mod Modulo 32.
• dup . Output on its own line non-destructively.
• + Add it to the sum.
• . Print the sum.