let's convert firstly the mixed fractions to improper fractions and then add up.
[tex]\bf \stackrel{mixed}{2\frac{3}{8}}\implies \cfrac{2\cdot 8+3}{8}\implies \stackrel{improper}{\cfrac{19}{8}}~\hfill \stackrel{mixed}{1\frac{1}{4}}\implies \cfrac{1\cdot 4+1}{4}\implies \stackrel{improper}{\cfrac{5}{4}} \\\\\\ \stackrel{mixed}{2\frac{7}{8}}\implies \cfrac{2\cdot 8+7}{8}\implies \stackrel{improper}{\cfrac{23}{8}} \\\\[-0.35em] ~\dotfill[/tex]
[tex]\bf \cfrac{19}{8}+\cfrac{5}{4}+\cfrac{23}{8}\implies \stackrel{\textit{using an LCD of 8}}{\implies \cfrac{(1)19+(2)5+(1)23}{8}}\implies \cfrac{19+10+23}{8} \\\\\\ \cfrac{52}{8}\implies \cfrac{13}{2}\implies 6\frac{1}{2}[/tex]
