Handle no Timer 3C

Marlin/src/HAL/HAL_AVR/fast_pwm.cpp

@@ -91,7 +91,7 @@ Timer get_pwm_timer(const pin_t pin) {
         #endif
       #endif
     #endif
-    #ifdef TCCR3A
+    #ifdef OCR3C
       case TIMER3C:   ++q;
       case TIMER3B:   ++q;
       case TIMER3A: {
@@ -103,6 +103,17 @@ Timer get_pwm_timer(const pin_t pin) {
         };
         return timer;
       }
+    #elif defined(OCR3B)
+      case TIMER3B:   ++q;
+      case TIMER3A: {
+        Timer timer = {
+          /*TCCRnQ*/  { &TCCR3A,  &TCCR3B,  nullptr },
+          /*OCRnQ*/   { &OCR3A,   &OCR3B,  nullptr },
+          /*ICRn*/      &ICR3,
+          /*n, q*/      3, q
+        };
+        return timer;
+      }
     #endif
     #ifdef TCCR4A
       case TIMER4C:   ++q;