There are two sorts of ENSEMBL ID. The first is the gene id. The gene MAPK3 maps to a single ENSEMBL gene id in human -
ENSG00000102882. The other sort of ID is the ENSEMBL transcript id. As MAPK3 has several transcripts, there are several ENSEMBL transcript ids.
Note that there are cases where a signle gene symbol has more than one ENSEMBL gene id. This is because HUGO (which decides gene symbols) and ENSEMBL (which assigned ENSEMBL ids) don't necessarily agree on what is what gene. So for example, the gene IGF2 has two ids: ENSG00000129965 and ENSG00000167244. This is because there is a read-through transcript that incorporates parts of both the classic IGF2 ORF and the adjacent INS ORF. Ensembl has decided this represents two different genes (IGF2 and INS-IGF2) where as HUGO only allocates a single SYMBOL (IGF2)