The first E in Mercedes sounds slightly different from the other two in German, mostly because the rhotic sound [r] modifies the tongue placement for the preceding E, forcing you to say it as either an open-mid front unrounded vowel [ɛ], or a mid near-front unrounded [ɛ̽]. The [r] prevents the vowel from being a Close-mid front unrounded vowel [e] like the 2nd and 3rd occurrences of E.

Or more simply, the first e sounds more like "bed" while the second and third sound more like "may", assuming you're reading this with a standard American dialect.