Infants and toddlers are not expected to be able to use all of the sounds that adults use in speech. However, there are certain sounds that most kids should be able to use with consistency. Here is a chart of some approximate ages that children should acquire sounds by (at the beginnings of words). 
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12 months: b, d, g, n, m, h
24 months: b, d, p, t, k, m, n, h, w
36 months: p, b, d, k, g, m, n, h, ch, w, l, "y" as in "yellow" and "th" as in "the"

McLeod, S., &; Bleile, K. (2003). From 2003 ASHA Convention: Neurological and developmental foundations of speech acquisition.

Here is what I like to see in 3 year-olds.

These are some ideas for things to work on/look at when your child is approaching his third birthday. Don't get too hung up on any one item, try to look at your child's speech from a wide view when they are really little (birth to 3).  For very young kids how they are using the words they have (language) is more important than how they are saying the words they have (articulation).


Sounds:
Children should be able to copy the “m”, “b”, “y”, “n”, “w”, “d”, “p”, and “h”
Children might be able to copy the “t”, “ng”, “k”, “g”, “f”, “v”, “ch”, and “j”


Words:
Children should use be using correct vowels in their words
Children should be able to say the sounds listed above in the beginning and endings of words (if they are alone and not in a group of consonants).
Children might not be able to say all of the consonants that are in a group (like the st in stop)
Children might replace a hard sound like a “v” with an easier sound like a “b”
Children should be talking with 3 or 4+ words in a phrase, expressing ideas, commenting, and requesting help.


Intelligibility:
Strangers should be able to understand around 75% of what child says (gestures included)
Parents should be able to understand around 90% of what child says (gestures included)

Here are some of my favorite Speech and Language Milestone References.

LinguiSystems Communication Milestones FREE (has a ton of data, might have some terms that are confusing if you are not super familiar with SLP literature)

Zero to Three Developmental Milestones a great resource for kids ages birth to 3 years hits all major areas of development (not just speech and language)

ASHA Birth to age 5 Reference  contains information about language and speech development for ages birth to five, broken down by how old they are

ASHA Kindergarten to 5th Grade Reference contains information about language skill for school age kids broken down by what grade they are in

There are a lot of developmental checklists out there for tracking your child's speech.  You will notice that each checklist has a wide range of when kids are supposed to be picking up new skills.  That is because all kids progress differently and that is ok...

sometimes it is hard to know exactly when to worry
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Will your child figure it out if you just watch and see?  Does your child need specialized help?  It can be confusing and cause anxiety...if you feel worried I suggest that you first talk with your pediatrician and the child's preschool or school teacher.  These professionals know normal development and are very good at sensing if there is a bigger problem.  Then you can reach out to a Speech and Language Pathologist (SLP) if it is appropriate.  There are some milestones that are very important for kids to master, a SLP will be able to tell you exactly which skills your child is in the process of learning and which will come next and where to start working and how to target each goal and most importantly...why these skills are so important.

​*tip - as you are researching I highly recommend checking out the American Speech-Language-Hearing Association website:  ASHA's website. It will contain the most recent evidence-based research so you can be sure you are getting facts and not crazy internet information that will just freak you out.