Training is on-going, programmed work designed to increase skill and/or capability. A critical component of training is measurement. Activity that fails to periodically measure skill and/or capability is not training.
Practice is skill or context-specific rehearsal. There is a component of practice in all training.
Instruction is receiving formally-organized information via a class or other means. This is very useful to anyone new to an idea or skill but ceases to be training when it fails to measurably improve skills.
Dictionaries do us a bit of disservice by sometimes listing these as synonyms but there is overlap. Receiving information via a formal class or similar means (instruction) might be the best training option, especially for a novice or someone new to a skill. For more skillful people, one-on-one skill consulting (coaching) is likely better. They don’t need new information about a skill (instruction), they need a knowledgeable person with a sharp eye for skill to observe what they already know and can do, and then offer an intelligent, organized path towards improvement. Rote repetitive rehearsal (practice) may be what’s needed to drive skill improvement. Much of this is best done by yourself and with a local peer group in regular, on-going sessions.
Instruction classes will likely involve some hands-on repetition (practice) of skills/techniques presented. A good instructor will also be able to work in a bit of one-on-one coaching during a class. There may also be an effort to establish an initial baseline of skills with the means to measure that. Besides presenting solid information, along with demonstrations and hands-on learning, the best thing a good class can offer a student is a clear path on what to do after the class is over. If the instructor didn’t hand you a program on what to do after the class along with a goal to work toward and the means to get there, the class and instructor is suspect.
Handgun presentation from a ready position, such as a holster, is a gun handling fundamental. This fundamental skill applies to any context where getting a firearm from ready to up and on target in a time-efficient manner is useful. Using this fundamental to win Steel Challenge events is a specific context and preparing properly for that specific context requires practice. Using this fundamental in a military environment is a different context but the skill remains the same.
Champion practical pistol competitor Doug Koenig discusses this here. Even when setting up to practice for a specific event, he continues to work base fundamental skills.
Hint: If a long-time national-level champion feels the need to continue working simple, fundamental skills as a primary component of his regimen, it is even more important for those of us with lesser skill to do so as well.