for the final cluster $($A$,$D$,$C$),$ how did they get distance $3\mathrm{.}6876$ for $($ADC$)$ to B

the way I calculated is by doing min$($dist$($A$,$B$),$ dist$($D$,$B$),$dist$($C$,$B$),$ which is $->$ min$(3\mathrm{.}6878,5\mathrm{.}0001,3\mathrm{.}5)->3\mathrm{.}5$