Valid:
Something, Foobar 111.

Invalid:
.......

validate :yourvalue_must_contain_at_least_3_alphabetic_characters

protected

def yourvalue_must_contain_at_least_3_alphabetic_characters
  unless yourvalue.gsub(/[^A-Z]/i,"").size > 2
    errors.add(:yourvalue, 'should have at least 3 alphabetic characters')
  end
end

validates_format_of :password, :with => /([^a-zA-Z]*([a-zA-Z]+)[^a-zA-Z]*){3,}/

protected

def validate
  unless your_field.gsub(/[^A-Z]/i,"").size > 2
    errors.add("your_field", "Must contain at least 3 alphabetical characters")
  end
end