Yes, if you don't expect to get the Social Security number for your third dependent before April 18, you can go ahead and file with the other two and then amend your return later.
You also have the option to file an extension, wait for the information and just file one return with all three dependents.
One thing you should know, even though you can e-file an amended return now, it still takes a long time for them to process it and issue a refund, typically at least 12 weeks. Click here to learn more about amending your return.
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