Simply use 'InputFormat' to parse the excel timestamp, and 'Format' to display it in your format of choice
% Made up data - you can parse an array in one call
excel_timestamp = ["12:37:48.123 AM"; "1:34:42.153 AM"; "3:25:31.635 AM"];
% Use the "InputFormat" & "Format" Name/Value pairs right at construction
datetime(excel_timestamp,"InputFormat","hh:mm:ss.SSS a", "Format", "hh:mm:ss.SSS")