Tharanga and Mendis struck half centuries as Sri Lanka beat Zimbabwe by six wickets to clinch the tri-series in Bulawayo on Sunday.
Zimbabwe beat West Indies by five runs under the D/L method in rain-affected match to qualify for the triangular series final against Sri Lanka.
Sri Lanka held their nerve to secure a one-run victory at Queens Sports Club on Wednesday and advance to the triangular series final.
Lightning and heavy rain caused fourth one-day international between Zimbabwe and Sri Lanka be abandoned after 13 overs.
Zimbabwe seamer Donald Tiripano bowled a superb final over to help his side earn a dramatic tie in their Tri-Series one-day international against West Indies in Bulawayo on Saturday.
Chasing a modest 226-run target, Sri Lanka failed to click with the bat and were bundled out for a paltry 165 in the tri-series contest in Harare on Wednesday.
Sri Lanka thrashed Zimbabwe by eight wickets in the first match of tri-series at Harare on Monday.
Zimbabwe take on Sri Lanka, in the first match of the tri-nation series, involving fellow visitors West Indies, at Harare on Monday.
Darren Bravo has been dropped from the West Indies squad in Zimbabwe for a tri-series because of "unacceptable behaviour", WICB said on Saturday.
All-rounder Kieron Pollard and wicketkeeper-batsman Denesh Ramdin have been dropped from the 15-man West Indies squad for next month's ODI tri-series in Zimbabwe.