By Martin Baccardax Slowing ad sales, particularly in YouTube, lead Google parent Alphabet to weaker-than-expected third quarter earnings. Alphabet (GOOGL) – Get Alphabet Inc. Report posted softer-than-expected third quarter Wednesday thanks to slowing ad sales growth that echoes a warning last week from messaging ap maker Snap (SNAP) – Get Snap Inc. Class A Report. Group revenues rose 6.1% from last year to $69.1 billion, a tally that soundly missed the Street consensus forecast of $70.6, as ad sales rose 2.5% to $54.48 billion. Search and other revenues rose 4.25% to $39.54 billion, just shy…