diff --git a/tests/Unit/StringPluralForm/ThreeFormsSlavicType1Test.php b/tests/Unit/StringPluralForm/ThreeFormsSlavicType1Test.php
new file mode 100644
index 0000000000000000000000000000000000000000..96301dd55206545b8f4950729dfa6f53b830c929
--- /dev/null
+++ b/tests/Unit/StringPluralForm/ThreeFormsSlavicType1Test.php
@@ -0,0 +1,92 @@
+<?php
+
+namespace Drupal\Tests\string_plural_form\Unit\StringPluralForm;
+
+use Drupal\string_plural_form\Plugin\StringPluralForm\ThreeFormsSlavicType1;
+
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\string_plural_form\Plugin\StringPluralForm\ThreeFormsSlavicType1
+ * @group StringPluralFormPlugins
+ */
+class ThreeFormsSlavicType1Test extends UnitTestCase {
+
+  /**
+   * Tests \Drupal\string_plural_form\Plugin\StringPluralForm\ThreeFormsSlavicType1::getGettextNplurals()
+   */
+  public function testGetGettextNplurals() {
+    $plugin = new ThreeFormsSlavicType1();
+    $this->assertEquals(3, $plugin->getGettextNplurals());
+  }
+
+  /**
+   * Tests \Drupal\string_plural_form\Plugin\StringPluralForm\ThreeFormsSlavicType1::getGettextNplurals()
+   */
+  public function testGetGettextPlural() {
+    $plugin = new ThreeFormsSlavicType1();
+    $this->assertEquals("n%10==1 && n%100!=11 ? 0 : n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2", $plugin->getGettextPlural());
+  }
+
+  /**
+   * Tests \Drupal\string_plural_form\Plugin\StringPluralForm\ThreeFormsSlavicType1::getIndex()
+   */
+  public function testGetIndex() {
+    $plugin = new ThreeFormsSlavicType1();
+    $expected_forms = [
+      0 => [
+        1, 21, 31, 41, 51, 61, 71, 81, 91,
+        101, 121, 131, 141, 151, 161, 171, 181, 191,
+      ],
+      1 => [
+        2, 3, 4,
+        22, 23, 24,
+        32, 33, 34,
+        42, 43, 44,
+        52, 53, 54,
+        62, 63, 64,
+        72, 73, 74,
+        82, 83, 84,
+        92, 93, 94,
+        102, 103, 104,
+        122, 123, 124,
+        132, 133, 134,
+        142, 143, 144,
+        152, 153, 154,
+        162, 163, 164,
+        172, 173, 174,
+        182, 183, 184,
+        192, 193, 194,
+      ],
+      2 => [
+        0, 5, 6, 7, 8, 9,
+        10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
+        20, 25, 26, 27, 28, 29,
+        30, 35, 36, 37, 38, 39,
+        40, 45, 46, 47, 48, 49,
+        50, 55, 56, 57, 58, 59,
+        60, 65, 66, 67, 68, 69,
+        70, 75, 76, 77, 78, 79,
+        80, 85, 86, 87, 88, 89,
+        90, 95, 96, 97, 98, 99,
+        100, 105, 106, 107, 108, 109,
+        110, 111, 112, 113, 114, 115, 116, 117, 118, 119,
+        120, 125, 126, 127, 128, 129,
+        130, 135, 136, 137, 138, 139,
+        140, 145, 146, 147, 148, 149,
+        150, 155, 156, 157, 158, 159,
+        160, 165, 166, 167, 168, 169,
+        170, 175, 176, 177, 178, 179,
+        180, 185, 186, 187, 188, 189,
+        190, 195, 196, 197, 198, 199,
+      ],
+    ];
+
+    foreach ($expected_forms as $index => $numbers) {
+      foreach ($numbers as $n) {
+        $this->assertEquals($index, $plugin->getIndex($n));
+      }
+    }
+  }
+
+}