Exhibit 99.3
 
 For immediate release
 Endeavour Announces $110 Million Offering of Convertible
Senior Notes
 Houston, July 18, 2011 
   This press release contains forward-looking statements within the meaning of Section 27A of
the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934. These
forward-looking statements include statements that express a belief, expectation, or intention, as
well as those that are not statements of historical fact, and may include projections and estimates
concerning the timing and success of specific projects and our future production, revenues, income
and capital spending. Our forward-looking statements are generally accompanied by words such as
estimate, project, predict, believe, expect, anticipate, potential, plan, goal or
other words that convey the uncertainty of future events or outcomes. We caution you not to rely
on them unduly. We have based these forward-looking statements on our current expectations and
assumptions about future events. While our management considers these expectations and assumptions
to be reasonable, they are inherently subject to significant business, economic, competitive,
regulatory and other risks, contingencies and uncertainties, most of which are difficult to predict
and many of which are beyond our control. These risks, contingencies and uncertainties, which may
not be exhaustive, relate to, among other matters, the following: discovery, estimation,
development and replacement of oil and gas reserves; decreases in proved reserves due to technical
or economic factors; drilling of wells and other planned exploitation activities; timing and amount
of future production of oil and gas; the volatility of oil and gas prices; availability and terms
of capital; operating costs such as lease operating expenses, administrative costs and other
expenses; our future operating or financial results; amount, nature and timing of capital
expenditures, including




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future development costs; cash flow and anticipated liquidity; availability
of drilling and production equipment; uncertainties related to drilling and production operations
in a new region; cost and access to natural gas gathering, treatment and pipeline facilities;
business strategy and the availability of acquisition opportunities; and factors not known to us at
this time. Any of these factors, or any combination of these factors, could materially affect our
future financial condition or results of operations and the ultimate accuracy of a forward-looking
statement. The forward-looking statements are not guarantees of our future performance, and our
actual results and future developments may differ materially from those projected in the
forward-looking statements. In addition, any or all of our forward-looking statements included in
this release may turn out to be incorrect. They can be affected by inaccurate assumptions we might
make or by known or unknown risks and uncertainties, including those included in our Annual Report
on Form 10-K for the year ended December 31, 2010. There can be no assurance that we will complete
the Marcellus acquisition.
 Endeavour International Corporation is an international oil and gas exploration and production
company focused on the acquisition, exploration and development of energy reserves in the North Sea
and United States. For more information, visit http://www.endeavourcorp.com
 For further information:
 Endeavour  Investor Relations
 Mike KirkseyDarcey Matthews
    
    713-307-8700
713-807-8711