In this case you have many to many Relationships between load and plane, load and truck, load and boat. I think its better if you use some type column to identify transport type. So you can do it using 3 tables now.
+---------------------------+
| load |
+---------------------------+
| load_id |
| |
+---------------------------+
+-------------+
| transport |
+-------------+
| id |
| type |
| |
+-------------+
+---------------------------+
| load_transport |
+---------------------------+
| load_id |
| transport_id |
+---------------------------+