from merriam-webster

Definition of STRINGENT
1: tight, constricted
2: marked by rigor, strictness, or severity especially with regard to rule or standard <stringent decontamination procedures>
3: marked by money scarcity and credit strictness <a stringent budget>

Definition of RIGOROUS
1: manifesting, exercising, or favoring rigor : very strict
2a : marked by extremes of temperature or climate b : harsh, severe
3: scrupulously accurate : precise

1archaic a : tight, close; also : intimate b : narrow
2a : stringent in requirement or control <under strict orders> b : severe in discipline <a strict teacher>
3a : inflexibly maintained or adhered to <strict secrecy> b : rigorously conforming to principle or a norm or condition
4: exact, precise <in the strict sense of the word>
5: of narrow

一点拙见：
stringent可以简单理解为 “严厉”，比如说 stringent fiscal policy，严厉的财政政策
后边的 rigorous 和 strict 都可以理解为 “严格”，区别嘛，我觉得 rigorous 更强调准确度，而strict更强调对于规矩的准守
比如，rigorous analysis, strict guidelines


但同时 M-W 给出来 stringent 的同义词，却又都包含了 rigorous, strict

Synonyms: brassbound, cast-iron, exacting, hard-line, inflexible, rigorous, strict, rigid, uncompromising

多读书，多体会吧

区别不大。去书店查英英字典，比较准确意思。

好强大