Speech, the ability to produce sounds and string them together to form words, is a complex task that requires development and maturity.  That being said, 3 year-old children typically are understood by unfamiliar adults 70-75% of the time, whereas the conversational speech of 4 year-olds is clearly produced closer to 85% of the time. 

Common speech errors might include “yeth” for “yes” and  “wabbit” for “rabbit.”  Omission of the beginning sound of a word is not typical for most ages. You might consider having him evaluated by a certified speech-language pathologist. This professional is qualified to determine whether or not your child's speech errors are developmental and can recommend home activities/suggestions and/or speech therapy. 
 
The same holds true for your concerns regarding your child's sentences.  Does this unusual word order occur each time he speaks, or only occasionally? Does it occur in particular situations?  Does it occur with specific relatives/friends?  It is helpful to  model the correct word order for him, and this is simple to do without drawing significant attention to it or to him. 

Consider contacting your local school district for more information regarding a speech-language evaluation for your son.