It may be in serious need of a tune-up. Timing and/or worn plugs can cause that. It may have a clogged fuel filter. It may have a bad fuel pump. The air filter may be clogged. You may have a vacuum leak. In a standard, you may have a worn clutch. In an automatic, if the transmission is in need of service, it could be as simple as a band adjustment that would be done when fluid and filter are changed, or it could be the transmission is not kicking down as it should. So many things to guess at, but, without seeing the truck myself, they are all guesses.
If it only happens in 3rd, I have to ask how fast you are going when this happens. Your speed may be low enough that the engine is just plain lugging, indicating you should be shifting down.
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