diff --git a/README.md b/README.md
index a3c0bc2..fd51557 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,7 @@
 
 # IrtRuby
 
-IrtRuby is a Ruby gem that provides implementations of the Rasch model and the Two-Parameter model for Item Response Theory (IRT). It allows you to estimate the abilities of individuals and the difficulties of items based on their responses to a set of items.
+IrtRuby is a Ruby gem that provides implementations of the Rasch model, the Two-Parameter model, and the Three-Parameter model for Item Response Theory (IRT). It allows you to estimate the abilities of individuals and the difficulties of items based on their responses to a set of items.
 
 ## Installation
 
diff --git a/lib/irt_ruby.rb b/lib/irt_ruby.rb
index 967ff42..a7e273c 100644
--- a/lib/irt_ruby.rb
+++ b/lib/irt_ruby.rb
@@ -3,6 +3,7 @@
 require "irt_ruby/version"
 require "irt_ruby/rasch_model"
 require "irt_ruby/two_parameter_model"
+require "irt_ruby/three_parameter_model"
 
 module IrtRuby
   class Error < StandardError; end
diff --git a/lib/irt_ruby/three_parameter_model.rb b/lib/irt_ruby/three_parameter_model.rb
new file mode 100644
index 0000000..cb5b1f2
--- /dev/null
+++ b/lib/irt_ruby/three_parameter_model.rb
@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+require "matrix"
+
+module IrtRuby
+  # A class representing the Three-Parameter model for Item Response Theory.
+  class ThreeParameterModel
+    def initialize(data)
+      @data = data
+      @abilities = Array.new(data.row_count) { rand }
+      @difficulties = Array.new(data.column_count) { rand }
+      @discriminations = Array.new(data.column_count) { rand }
+      @guessings = Array.new(data.column_count) { rand * 0.3 }
+      @max_iter = 1000
+      @tolerance = 1e-6
+    end
+
+    def sigmoid(x)
+      1.0 / (1.0 + Math.exp(-x))
+    end
+
+    def probability(theta, a, b, c)
+      c + (1 - c) * sigmoid(a * (theta - b))
+    end
+
+    def likelihood
+      likelihood = 0
+      @data.row_vectors.each_with_index do |row, i|
+        row.to_a.each_with_index do |response, j|
+          prob = probability(@abilities[i], @discriminations[j], @difficulties[j], @guessings[j])
+          if response == 1
+            likelihood += Math.log(prob)
+          elsif response.zero?
+            likelihood += Math.log(1 - prob)
+          end
+        end
+      end
+      likelihood
+    end
+
+    def update_parameters
+      last_likelihood = likelihood
+      @max_iter.times do |_iter|
+        @data.row_vectors.each_with_index do |row, i|
+          row.to_a.each_with_index do |response, j|
+            prob = probability(@abilities[i], @discriminations[j], @difficulties[j], @guessings[j])
+            error = response - prob
+            @abilities[i] += 0.01 * error * @discriminations[j]
+            @difficulties[j] -= 0.01 * error * @discriminations[j]
+            @discriminations[j] += 0.01 * error * (@abilities[i] - @difficulties[j])
+            @guessings[j] += 0.01 * error * (1 - prob)
+          end
+        end
+        current_likelihood = likelihood
+        break if (last_likelihood - current_likelihood).abs < @tolerance
+
+        last_likelihood = current_likelihood
+      end
+    end
+
+    def fit
+      update_parameters
+      {
+        abilities: @abilities,
+        difficulties: @difficulties,
+        discriminations: @discriminations,
+        guessings: @guessings
+      }
+    end
+  end
+end
diff --git a/spec/irt_ruby/three_parameter_model_spec.rb b/spec/irt_ruby/three_parameter_model_spec.rb
new file mode 100644
index 0000000..58cbd4c
--- /dev/null
+++ b/spec/irt_ruby/three_parameter_model_spec.rb
@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe IrtRuby::ThreeParameterModel do
+  let(:data) { Matrix[[1, 0, 1], [0, 1, 0], [1, 1, 1]] }
+  let(:model) { IrtRuby::ThreeParameterModel.new(data) }
+
+  describe "#initialize" do
+    it "initializes with data" do
+      expect(model.instance_variable_get(:@data)).to eq(data)
+    end
+  end
+
+  describe "#sigmoid" do
+    it "calculates the sigmoid function" do
+      expect(model.sigmoid(0)).to eq(0.5)
+    end
+  end
+
+  describe "#probability" do
+    it "calculates the probability with guessing parameter" do
+      expect(model.probability(0, 1, 0, 0.2)).to be_within(0.01).of(0.6)
+    end
+  end
+
+  describe "#likelihood" do
+    it "calculates the likelihood of the data" do
+      expect(model.likelihood).to be_a(Float)
+    end
+  end
+
+  describe "#fit" do
+    it "fits the model and returns abilities, difficulties, discriminations, and guessings" do
+      result = model.fit
+      expect(result[:abilities].size).to eq(data.row_count)
+      expect(result[:difficulties].size).to eq(data.column_count)
+      expect(result[:discriminations].size).to eq(data.column_count)
+      expect(result[:guessings].size).to eq(data.column_count)
+    end
+  end
+end
diff --git a/spec/irt_ruby/two_parameter_model_spec.rb b/spec/irt_ruby/two_parameter_model_spec.rb
index 0ef61e9..0ccd07a 100644
--- a/spec/irt_ruby/two_parameter_model_spec.rb
+++ b/spec/irt_ruby/two_parameter_model_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "spec_helper"
 
 RSpec.describe IrtRuby::TwoParameterModel do