Charge of an amino acid sequence
When the pH is lower than the pKa, the group will be protonated, and when the pH is higher than the pKa, the group will be deprotonated. Your explanation is correct that basic groups will be positively charged when protonated and neutral when deprotonated and that acidic groups will be neutral when protonated and negatively charged when deprotonated. Shown below is the compound in it's fully protonated state with the given pKa's.*



The given pH is 3.3 which means that:


N-terminus: pH &lt; pKa, protonated basic group, +1
lys-sidechain: pH &lt; pKa, protonated basic group, +1
asp-sidechain: pH &lt; pKa, protonated acidic group, 0
his-sidechain: pH &lt; pKa, protonated basic group, +1
arg-sidechain: pH &lt; pKa, protonated basic group, +1
C-terminus: pH > pKa, deprotonated acidic group, -1


The overall charge is +3.

*An aside, the pKa's for the side chains are a gross approximation here. It turns out they are sufficient to answer the question, but given a higher pH, the answer wouldn't be correct.