UPDATE 1-Knight Capital appoints three new directors

* TD Ameritrade CEO Fred Tomczyk joined Knight's board as of Monday

* Blackstone Group's Martin Brand and General Atlantic's Matthew Nimetz also added

NEW YORK Aug 28 (Reuters) - Financial services company Knight Capital Group Inc said on Tuesday that it had appointed three new board members, including the chief executive officer of TD Ameritrade Holding Corp, three weeks after receiving a $400 million investment from a group of firms to keep it afloat.

Technical errors at Knight on Aug. 1 caused unintended trades that punched a $440 million hole in its balance sheet.

A group of investors including Blackstone Group, Getco and financial services companies TD Ameritrade, Stifel Nicolas , Jefferies Group Inc and Stephens Inc rescued the embattled market maker in a deal that kept it in business.

Fred Tomczyk, 56, the CEO of TD Ameritrade, joined Knight's board as of Monday, Knight said.

Martin Brand, 37, the managing director of Blackstone Group LP, and Matthew Nimetz, 73, advisory director of General Atlantic LLC, also joined Knight's board. The appointments were reported earlier by the Wall Street Journal.

The additions increase the number of directors to 10 at Knight, which is one of the two biggest executors of stock trades in the United States.