using System;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
        var re = new Regex(@"\b[аеоиуюэяёы][а-яё][аеоиуюэяёы]\b",
                           RegexOptions.IgnoreCase | RegexOptions.Compiled);
 
		string input;
		string text = "";
		while ((input = Console.ReadLine()) != null) text += "\n"+input;
		Console.WriteLine($"Input text: {text}");
        foreach (Match match in re.Matches(text))
            Console.WriteLine($"  --> match at index {match.Index}, length {match.Length}, text {match.Value}");
	}
}

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0LDQs9CwCtC+0LPQvgrQvdC10YIK0YLRgNC4CtGH0LXRgtGL0YDQtQpxd2UK0LDQs9CwINC+0LPQviDRg9Cz0YMg0YXRgNGOINCz0LDQsiDQvNGP0YMg0YLQv9GA0YMg0L3QviDRi9GL0Ys=

ага
ого
нет
три
четыре
qwe
ага ого угу хрю гав мяу тпру но ыыы