How can I Count every word with three or more syllables in each

The only way you can do this is by using a database of word-syllable. Here's one way using www.dictionary.com as the database.

FullText = 'This is a really arbitrary sentence.'; 
%'really' could be pronounced 'ree-uh-lee' (3 syl)

TextCell = regexp(FullText, '\w+', 'match'); 
TextSyl = cellfun(@(x) getSyllable(x), TextCell);
TextSyl =
     1     1     1     3     4     2  

OneSylWord = sum(TextSyl == 1);
OneSylWord = 
     3

Where getSyllable function is:

function Syl = getSyllable(Word)

if nargin == 0
    Word = input('What word do you want? ', 's');
end
if isempty(Word)
    Syl = 0;
    return
end
Word = strrep(Word, ' ', '');

% Use dictionary.com to get the phonetic transcription of a word
%  Ex: arbitrary
%    [ahr-bi-trer-ee]
% WARNING: will not work for some words if dictionary.com does not have it
%  listed as the main word. Example, 'awesomeness' returns 2 because
%' awesome' is the main word in the site.

try
    SiteTxt = urlread(sprintf('http://www.dictionary.com/browse/%s?s=t', Word));
catch
    warning('Could not determine syllable for "%s". Returning 0.', Word);
    Syl = 0;
    return
end

CodeSrch1 = '"pron spellpron"[\s\w\d\>]+\[\s*(?[^\]]+)';
InnerCode = regexp(SiteTxt, CodeSrch1, 'tokens');
InnerCode = InnerCode{1};

CodeSrch2 = '>(?[^\<]+)';
Phonetics = regexp(InnerCode, CodeSrch2, 'tokens');
Phonetics = [Phonetics{1}{:}];
if isempty(Phonetics)
    Syl = 1;
else
    Phonetics = cat(2, Phonetics{:});
    MultWord = regexp(Phonetics, ',', 'split'); %Sometimes many ways to say a word - take 1st option
    Syl = sum(MultWord{1} == '-') + 1;
end