Why B

It's diminished because unlike the others, it is composed of a minor third plus a DIMINISHED FIFTH.
It contains the notes B,D & F. B to D is a minor third and B to F is a diminished fifth.
All the other chords contain either a major or minor third plus a PERFECT FIFTH.

Another way to look at it is that it is composed of two stacked minor thirds.
B to D is a minor third and D to F is also a minor third.
By contrast, Major chords contain a major third plus a minor third, and minor chords contain a minor third plus a major third

Table
M3 (Major 3rd) + m3 (minor 3rd) = Major chord
m3 + M3 = minor chord
m3 + m3 = diminished chord
M3 + M3 = augmented chord (not present in major scales, but present in altered minor scales)

but how would a m3 look on the guitar. whould it just belike an A or something. i think i'm just not procesing this right.

M3 (Major thirds) and m3 (minor thirds) are simple two-note intervals of a fixed size.
M3 = 4 half steps (semitones)
m3 = 3 half steps

In the chord you mentioned (B diminished), the notes are B D & F
The distance from B to D is an interval of a minor third, and D to F is also an interval of a minor third.

Here is a simple 3 note B dim chord (triad) on strings 1,2 & 3
Frets: XXX431 or XXXBDF

So just playing strings 3 and 2 give you a minor third interval.
Playing strings 2&1 (D & F) also gives you a minor third interval.
Playing both intervals at the same time (i.e., all 3 strings) gives you the chord B diminished

Look at this page and it will show how the dim chord comes about.

C-major: C D E F G A B

C E G=C
D F A=Dm
E G B=Em
F A C=F
G B D=G
A C E=Am
B D F=Bdim

B D# F#=B
B D F#=Bm
B D F=Bdim

As a general rule: the seventh chord in every major scale is diminished, the formula is:
M-m-m-M-M-m-dim

So for example in A-major:
A B C# D E F# G#

With the formula:
A Bm C#m D E F#m G#dim

THanks you guys. You all rock. When my band goes big, I'll mention you all in my grammy speach :D :lol: :lol: