documentclass{article}
usepackage{longtable}
usepackage{tabu}
usepackage{booktabs}
begin{document}
begin{longtabu}[c]{lrX}
caption{Some table}
bfseries Column 1 & bfseries Column 2 & bfseries Column 3%
midrule
endhead
end{longtabu}
end{document}

! Extra alignment tab has been changed to cr.
<template> endtemplate

l.11 end{longtabu}